Athena acronym list, actively maintained by Jos de Bruijne (with input from Matthias Ehle). A useful X-ray glossary maintained by the XMM-Newton Science Operations Centre can be found here. Additions, corrections, and feedback on the below list is very welcome.
 
Last updated 24 August 2020, containing 1675 entries.
 
1g : Earth's gravity
3E : Electrical, Electronic, and Electro-mechanical, sometimes referred to as triple E (components)
A : Abell (galaxy cluster)
A : Acceptance (testing)
A : Analysis
A64 : Ariane-6.4 (launcher)
AA : Alignment Acceptance
AAD : Attitude-Anomaly Detector
AAP : Active-Archival Phase
AAT : Advanced Analysis Tool
AB : Abstract
ABCL : As-Built Configuration List
ABS : ABSolute
AC : Active Cascode
AC : Alternating Current
AC : Analytical Calculator (within SIMPOSiuM)
AC : Anti-Coincidence
ACC : ACCeleromter
ACD : Anti-Coincidence Detector
ACE : Advanced Composition Explorer (spacecraft)
ACO : Athena Community Office
ACP : Athena Calibration Plan
ACPTF : Athena Calibration Plan Task Force
ACS : Activity Control Sheet
ACSS : Athena Common Software System
ACSST : Athena Common Software System Team
AD : Adjusted Derivative (pulse detection)
AD : Analogue to Digital
AD : Applicable Document
ADA : Athena Data Analysis
ADC : Analogue-to-Digital Converter
ADE : Actuator Drive Electronics
ADF : Attitude Determination Frame
ADR : Adiabatic Demagnetisation Refrigerator
ADS : Airbus Defence and Space
AE : ArianEspace
AE : Astro-E
AE8/AE9 : NASA trapped electron/proton environment models
AFE : Analogue Front End
AFM : Atomic-Force Microscope
AFT : Abbreviated Functional Test
AGH : Akademia G\’{o}rniczo-Hutnicza (University of Science and Technology, Krak\’{o}w)
AH : Astro-H
AHEAD : (Integrated) Activities in the High Energy Astrophysics Domain
AHEPaM : Athena High-Energy Particle Monitor
AIB : Analogue Interface Board
AIT : Assembly, Integration, and Test
AIV : Assembly, Integration, and Verification
AKE : Absolute Knowledge Error
AL : Acceptance (test) Load
ALAT : Air Liquide Advanced Technologies
ALBA : synchrotron radiation facility in Barcelona, Spain (meaning “sunrise”)
ALD : Atomic Layer Deposition
ALM : Additive Layer Manufacturing
AMA : Athena Mirror Assembly
AMBER : Active Monitor Box of Electrostatic Risk (radiation monitor)
AMOLF : (instituut voor) Atoom- en MOLecuulFysica (institute for atomic and molecular physics)
AMS : Alpha Magnetic Spectrometer (ISS)
AMXP : Accretion-powered Millisecond X-ray Pulsar
AO : Announcement of Opportunity
AOB : All Other Business
AOCS : Attitude and Orbit Control Sub-system
AOS : Acquisition Of Signal
AP : As Proposed
APC : AstroParticule and Cosmologie (Paris)
APE : Absolute Performance (Pointing) Error
APEC : Astrophysical Plasma Emission Code
APM : Ammonia Peroxide Mix
APM : Antenna Pointing Mechanism
APME : Antenna Pointing Mechanism Electronics
APR : Array Power Regulator
APS : Active Phase Shift(er)
APS : Active-Pixel Sensor
APSW : APplication SoftWare
AR : Acceptance Review
AR : Annual Review
AR : Anomaly Review
AR : Aspect Ratio (pixel)
AR64 : Ariane-6.4 (launcher)
ARB : Acceptance Review Board
ARB : Anomaly Review Board
ARBEB : Athena Red Book Editorial Board
ARCUS : Arcus MIDEX Explorer (NASA)
AREMBES : Athena Radiation Environment Models and X-Ray Background Effects Simulators (Athena Radiation Environment Modelling Background EStimator)
ARES : Analysis and Reporting System
ARF : Ancillary Response File
ARSE : Angular Resolution Sensitivity Exercise
ASA : Athena Science Archive
ASCA : Advanced Satellite for Cosmology and Astrophysics
ASCII : American Standard Code for Information Interchange
ASDC : Athena Science Data Centre
ASE : Athena System Engineering (Board)
ASF : AREMBES Simulation Framework
ASGS : Athena Science Ground Segment
ASI : Agenzia Spaziale Italiana
ASIC : Application-Specific Integrated Circuit
ASIE : Athena Science Impact Exercise
ASMB : Athena System Management Board
ASPHEA : Alignment of Silicon Pore optics for High-Energy Astronomy
ASSATF : Athena Science Sensitivity Analysis Task Force
ASST : Athena Science Study Team
ASST : Athena Software and System Team
AST : Analysis STep
ASW : Application Software
AT : AusTria
ATAC : Athena Time Allocation Committee
ATB : Avionics Test Bench
ATHENA : Advanced Telescope for High-ENergy Astrophysics
ATP : Authorisation To Proceed
ATSAL : ATomic Spectral AnaLysis
AV : Advanced Variant
AVM : AVionics Model
AWG : Astronomy Working Group (ESA)
AXB : Astrophysical X-ray Background
Al : Aluminium
ApC : Aperture Cylinder
B : Belgium
B-H : magnetic induction and magnetic field
B2CD : project phases B2, C, and D combined
B4C : Boron_4-Carbide
BAF : BAFfle
BAM : Background Analysis Module (SPM/WFI)
BAR : Background Assessment Report
BAT : BATtery sub-system
BB : Bread Board
BBFB : BaseBand FeedBack
BCB : BenzoCycloButene (coating)
BCDR : Battery Charge and Discharge Regulator
BCG : Brightest Cluster Galaxy
BCM : Battery Charge Management
BCR : Battery Charge Regulator
BD : Brown Dwarf
BDR : Battery Discharge Regulator
BDRF : Bi-Directional Reflectivity Function
BE : Background Estimate
BEAR : Bending magnet for Emission, Absorption, and Reflectivity (beam line)
BEE : Back-End Electronics
BERT : BERTini cascade (model)
BESSY : Berliner ElektronenSpeicherring-gesellschaft f\"{u}r SYnchrotronstrahlung
BEUR : Billion EURo
BEaTriX : Beam Expander Testing X-ray facility
BF : Best Focus
BF : Blocking Factor
BGD : BackGrounD
BH* : Stellar-mass Black Hole
BHB : Black-Hole Binary 
BHC : Black-Hole Candidate
BIC : BInary Cascade (model)
BIPR : Background Intellectual Property Right
BISPO : Baffled IXO SPO (mirror module)
BKG : BacKGround
BL : Boundary Layer
BLWN : Band-Limited White Noise 
BND : Beam-Normalisation Detector
BOT : Beginning Of Track
BP : Base Plate
BS : BremsStrahlung
BSC : Back-SCattering
BSCW : Basic Support for Cooperative Work
BSE : Back-Scattered Electron
BSM : Bi-level Standard Monitor
BUSD : Billion US Dollar
BUT : Brno University of Technology
B_4C : Boron_4-Carbide 
BoE : Back of (the) Envelope (calculation)
BoGEMMS : Bologna GEant4 Multi-Mission Simulator
BoL : Beginning of Life
Br : Bracket (spectral line)
C : (degree) Celsius
C : Capacitor
C : Carbon
C&DH : Command and Data Handling
C/B : Clamp Band
CA : Conical(-approximation) Athena
CAD : Computer-Aided Design
CAF : Contiguous Area Fraction
CAP : Cooler Ancillary Panel
CAS : Calibration ASsembly (X-IFU)
CAS : Czech Academy of Sciences
CATIA : Computer-Aided Three-dimensional Interactive Application
CBE : Current Best Estimate
CBK : Centrum Bada\'{n} Kosmicznych
CBUSH : specific element in NASTRAN
CC : Cleanliness and Contamination
CC : Cold Core (galaxy cluster)
CC : Cooling Chain
CC : Core Collapse (supernova)
CC : Cost-Constrained (mission)
CCB : Configuration Control Board
CCCM : Channel-Cut Crystal Monochromator
CCCP : Cleanliness, Contamination, and Control Plan
CCD : Charge-Coupled Device
CCD : Contract Closure Document
CCD : Contract-Closure Documentation
CCDR : Closed-Cycle Dilution Refrigerator
CCE : Cooler Control Electronics
CCGA : Ceramic Column Grid Array
CCH : Cooler Cryo(genic) Harness
CCM : Centre for Concepts in Mechatronics
CCN : Change-of-Contract Notice
CCP : Component Control Plan
CCP : Contamination Control Plan
CCSDS : Consultative Committee for Space Data Systems
CCU : Central Control Unit
CCU : Cryo(genic) Cooler Unit
CD : Compact Disc
CD : Cool Down
CDA : Clean Dry Air
CDE : Cooler Drive Electronics
CDF : Concurrent Design Facility (ESA)
CDF : Cumulative Distribution Function
CDFS : Chandra Deep Field South
CDM  : Code-Domain Multiplexing (Code-Division Multiplexing)
CDMS : Command and Data Management Sub-system
CDMU : Central Data Management Unit
CDR : Critical Design Review
CE : Conducted Emission
CE : Conformit\'{e} Europ\'{e}enne
CEA : Commissariat à l'\'{E}nergie Atomique
CEB : CEBreros (35-m ESA ground station near Madrid, Spain)
CELAS : specific element in NASTRAN
CF : Crest Factor
CFDP : CCSDS File Delivery Protocol
CFE : Customer-Furnished Equipment
CFEE : Cold Front-End Electronics
CFI : Customer-Furnished Item
CFRP : Carbon-Fibre-Reinforced Plastic
CG : Cut Gain
CGH : Computer-Generated Hologramme
CH : Camera Head (WFI)
CH : La CHuisse (Switzerland)
CHT : Cylindrical Hall Thruster
CHX : Cooler Heat eXchange(r)
CI : Configuration Item
CI : Conformance Inspection
CIC : Connector Identification Code
CIDL : Configuration Item Data List
CIL : Critical Items List
CIRSOS : Collaborative Iterative Radiation Shielding Optimisation System
CLA : Cooler (connecting) Line Assembly
CLA : Coupled-Load Analysis
CLS : Canadian Light Source
CM : Common Mode
CM : Compliance Matrix
CM : Consortium Meeting
CM : Ministerial Council (meeting)
CMA : Cooler Mechanical Assembly
CMCU : Common Mode Correction Unit
CME : Coefficient of Moisture Expansion
CME : Coronal Mass Ejection
CMM : Coordinate Measuring Machine
CMRR : Common-Mode Rejection Ratio
CMZ : Central Molecular Zone
CN : Change Notice
CN : Classical Nova
CNC : Computer Numerical Controlled
CND : CircumNuclear Disk
CNES : Centre National d'\'{E}tudes Spatiales
CO : Chief Officer
CODEV : Code V optics / ray-tracing simulation software
COM : COMmunication sub-system
COMSOL : COMputer SOLution (finite-element analysis, solver, and multi-physics simulation software)
CORE : Cost-Oriented Reprogramming Exercise (Cost-driven Observation Reprogramming Exercise)
COSPAR : Committee On SPAce Research
COTS : Commercial Off-The-Shelf
COVID(-19) : COrona VIrus Disease (2019)
CP : Calibration Plan
CP : Commissioning Phase
CPA : Cooler Compressor Assembly
CPD : Charged-Particle Diverter
CPF : Complex Pupil Function
CPM : Central Processing Module
CPS : Central Plasma Sheet
CPS : Chemical Propulsion (Propellant) Sub-system
CPS : Counts Per Second
CR : Change Request
CR : Clean Room
CR : Cosine Research
CR : Cosmic Ray
CR : Count Rate
CR : Czech Republic
CRD : Calibration Requirements Document
CRS : Cleanliness Requirement Specification
CRS : Coarse Rate Sensor
CRV : (time-)Constant Random Variable
CS : Conductive Susceptibility
CS: Cooling System
CSD : Coordinate System Document
CSI : Customer Source Inspection
CSL : Centre Spatial de Li\`{e}ge
CSS : Core Science Survey
CSV : Comma-Separated Values
CSW : Central SoftWare
CT : Compton Thick
CT : Core Test
CTA : Cherenkov Telescope Array
CTE : Coefficient of Thermal Expansion
CTP : Core Technology Programme (ESA)
CTQ : Critical To Quality
CTR : Central Time Reference
CTTS : Classical T-Tauri Star
CV : Conservative Variant
CV : Cosmic Vision (programme of ESA's science directorate)
CVCM : Collected Volatile Condensable Material
CWB : Colliding-Wind Binary
CX : Charge eXchange
CXB : Cosmic X-ray Background
CXC : Chandra X-ray observatory Center
CXO : Chandra X-ray Observatory
CaC : Cost at Completion
CaCC : Cost-at-Completion Cap
CeSiC : Carbon-fiber-reinforced Silicon-Carbide
Cfg : Configuration
CoB : (X-IFU) Consortium Board
CoB : Close of Business
CoC : Certificate of Conformance
CoDR : Conceptual Design Review
CoG : Centre of Gravity
CoM : Centre of Mass
CoP : Centre of Pressure
ConOps : Concept of Operations
D : Design
D : Diplexer
DAC : Digital-to-Analogue Converter
DAQ : Data AQuisition
DBRR : DCS Breadboarding for Risk Reduction
DC : Direct Current
DC : Double Cone
DCCM : Document Control and Change Manager
DCL : Declared Components List
DCM : Double-Crystal Monochromator
DCS : Demonstrator of the (X-IFU) Cooling System
DCS : Detector Cooling System
DD : (X-IFU) Dewar Door
DD : Double Degenerate
DDF : Design Definition File
DDOR : Delta Differential One-way Ranging
DDT : Director’s Discretionary Time
DDVP : Design, Development, and Verification Plan
DE : Detector Electronics
DEE : DE-Excitation Model
DEPFET : DEpleted P-channel Field-Effect Transistor
DET : Direct Energy Transfer
DF : De-Focused
DHS : Data-Handling Sub-system
DI : De-Ionised
DIL : Deliverable Item List
DIW : Deionised Water
DJF : Design Justification File
DK : DenmarK
DLC : Diamond-Like Carbon
DLL : Design Launch Load
DLL : Design Limit Load
DLR : Deutsches Zentrum für Luft- und Raumfahrt
DM : Demonstration Model
DM : Differential Mode
DML : Declared Materials List
DMM : Design Maturity Margin
DMM : Dynamic Mathematical Model
DMMPL : Declared Materials And Processes and Mechanical Parts List
DMPL : Declared Mechanical Parts List
DMSO : DiMethyl SulfurOxide
DN : Dwarf Nova
DNEL: Disable Non-Essential Loads
DOORS : Dynamic Object Oriented Requirements System
DP : Data Pack
DP : Data Processing
DP : Decommissioning Phase
DP : Detector Plane
DP : Disposal Phase
DP : Dowel Pin
DPA : Destructive Physical Analysis
DPL : Declared Processes List
DPR : Detector-Plane Response
DR : DRain current (readout mode)
DRB : Delivery Review Board
DRD : Document Requirements Description (Definition)
DRE : Digital Read-out Electronics
DRL : Deliverable (Documents) Requirements List
DS : Death Star (MM assembly jig)
DS2 : Death Star version 2 (MM assembly jig version 2)
DSL : Domain Specific Language
DSMC : Direct Simulation Monte-Carlo
DST : Deep Space Transponder
DT : Dead Time
DT : Discretionary Time
DTA : Detent high-Torque rotary Actuator
DTCP : Daily TeleCommunications Period
DTU : DTU Space (formally: National Space Institute, Danish: Institut for Rumforskning og Rumteknologi), part of the Technical University of Denmark (Danmarks Tekniske Universitet)
DUL : Design Ultimate Load
DV : Delta V(elocity)
DWV : Dielectric withstanding voltage
DYL : Design Yield Load
DoD : Depth of Discharge
DoF : Degree of Freedom
E : Espana (Spain)
E2E : End-to-End
E2EPA : End-to-End Performance Assessment
EA : Effective Area
EAST : ESA Athena Science Team
EAST : ESO-Athena Synergy Team
EBIT : Electron Beam Ion Trap
ECAP : Erlangen Centre for Astro-particle Physics
ECC : ESTRACK Control Centre
ECR : Earth-Centred Rotating (coordinate system)
ECS : EBIT Calorimeter Spectrometer
ECS : Environmental Control Sub-system
EDM : Electrostatic Discharge Machining
EEE : Electrical, Electronic, and Electromechanical (triple-E)
EF : Eigen Frequency
EFM : ESTRACK Facilities Manual
EFM : Electrical Functions Model
EFR : Established Failure Rate
EFU : Event Filter Unit
EGSE : Electrical Ground-Support Equipment
EIA : Electronic Industries Alliance
EID : Experiment Interface Document
EID-A : Experiment Interface Document part A
EID-B : Experiment Interface Document part B
EIDP : End-Item Data Package
EIPS : Electron Impact Point Source
EIRP : Equivalent Isotropic Radiated Power
ELG : Event List Generator
ELP : Low-Energy Proton
ELR : End-of-Life Review
EM : Electrical Model
EM : Emission Measure
EM : Engineering Model
EMC : Electro-Magnetic Compatibility
EMI : Electro-Magnetic Interference
EMV(X) : Variation of Standard EM package
ENC : Equivalent Noise Charge
EOP : Extended Operational Phase
EOT : End Of Track
EP : Event Processing
EPC : Electronic Power Conditioner
EPHIN : Electron Proton Helium INstrument
EPIC : Energetic Particles and Ion Composition
EPIC : European Photon Imaging Camera
EPPL : European Preferred Parts List
EPS : Electrical Power Sub-system
EQM : Engineering Qualification Model
EQSOL : EQuipment Switch-Off Line
EQSR : Equipment Qualification Status Review
ERB : Energy-Resolution Budget
ERM : EPIC Radiation Monitor
ERT : Earth Reception Time
ES : Electronic Structure
ES : Energy Scale
ESA : European Space Agency
ESAC : European Space Astronomy Centre (Villafranca, Spain)
ESAM : Emergency Safe Attitude Mode
ESC : Electro-Static Chuck
ESCC : European Space Components Coordination
ESD : ElectroStatic Discharge
ESDC : ESAC Science Data Centre
ESHIEM : Energetic Solar Heavy Ion Environment Modelling
ESMDB : European Space Materials Database
ESOC : European Spacecraft Operations Centre (Darmstadt, Germany)
ESR : Equivalent Serial Resistance
ESR : Executive Summary Report
ESTEC : European Space research and TEchnology Centre (Noordwijk, The Netherlands)
ESTRACK : ESA Tracking Stations Network
ESU : Energy Storage Unit
ET : Einstein Telescope
ETB : Electrical Test Bench
EU : Electronics Unit
EU : European Union
EUV : Extreme UltraViolet
EVT : Engineering Validation Test
EW : Equivalent Width
EXACRAD : Experimental Evaluation of Athena Charged Particle Background from Secondary Radiation and Scattering in Optics
EXAFS : Extended X-ray Absorption Fine Structure
EoL : End of Life
EoR : Epoch of Reionisation 
F : (degree) Fahrenheit
F : La France
F2F : Face-to-Face
F2F : Face-to-Face (meeting)
FAR : Final Acceptance Review
FAR : Flight Acceptance Review
FAT : Factory Acceptance Test
FAU : Friedrich Alexander University
FC : (WFI) Fast Chip (obsolete, now called FD)
FCM : Four Crystal Monochromator
FD : (WFI) Fast Detector
FDIR : Failure Detection, Isolation, and Recovery
FDM : Frequency-Domain Multiplexing
FDS : Flight-Dynamics System
FE : Finite Element (model)
FE : Front End
FEA : Finite-Element Analysis
FEE : Front-End Electronics
FEM : Finite-Element Model
FER : Frame-Error Rate
FF : Full Frame
FFT : Fast Fourier transform
FFT : Full Functional Test
FGS : Fine Guidance Sensor
FGSE : Fluidic Ground-Support Equipment
FI : Finland
FINREF : FINancial REForm (framework for costing of ESA activities)
FIR : Facility Implementation Review
FITS : Flexible Image Transport System
FL : Focal Length
FLL : Flux-Locked Loop
FM : Flight Model
FMC : FPGA Mezzanine Card
FMEA : Failure Modes and Effects Analysis
FMECA : Failure Modes, Effects, and Criticality Analysis
FMS : Fixed Metering Structure
FOS : Factor Of Safety
FOSU : Ultimate design Factor Of Safety
FOSY : Yield design Factor Of Safety
FP : Final Presentation
FP : Flight Programme
FPA : Focal-Plane Assembly
FPC : Flow-Proportional Counter
FPCS : Focal-Plane Coordinate System
FPGA : Field-Programmable Gate Array
FPM : Focal-Plane Module (obsolete, now called SIM)
FPM : Frame-Processing Module (WFI)
FPP : Focal-Plane Platform (obsolete, now called SIP)
FPR : Focal-Plane Response
FPT : Full Performance Test
FR : Final Report
FR : Final Review
FR-II : Faranoff-Riley (type-2) radio galaxy
FRM : Fringe Reflection Metrology
FRR : Flight Readiness Review
FRT : Fringe-Reflection (metrology) Technique
FRW : Request for Waiver
FS : Feasibility Study
FS : Flight Spare (Model)
FTA : Fault Tree Analysis
FTF : FriTioF (model)
FTIR : Fourier Transform InfraRed
FVI : Functional Verification Infrastructure
FW : Filter Wheel
FW&F : Filter Wheel and Filter
FWA : Filter-Wheel Assembly
FWV : Windowed Variance Filter
FY : Fiscal Year
FeKa : Iron K_\alpha (line)
FoM : Figure of Merit
FoR : Field of Regard
FoV : Field of View
FuMo : Functional Model
G4 : GEANT4
GAPHE : Groupe AstroPhysique des Hautes Energies (Li\`ege)
GBH : Galactic Black Hole
GCR : Galactic Cosmic Ray
GDIR : General Design and Interface Requirements
GDML : Geometry Description Markup Language
GEANT : GEometry ANd Tracking
GEANT4 : GEometry ANd Tracking version 4
GEO : Geostationary Earth Orbit
GIM : Galvanic Isolation Module
GM : Gifford-McMahon (cooler)
GMM : Geometrical Mathematical Model
GMSK : Gaussian Minimum Shift Keying
GMT : Greenwich Mean Time
GNC : Guidance and Navigation Control
GND : GrouND(ing)
GO : Guest Observer
GPS : Global Positioning System
GRAS : GEANT4 Radiation Analysis for Space
GRB : Gamma-Ray Burst
GRP : Giant Radio Pulse
GRS : Geodetic Reference System
GS : Ground Segment
GS : Ground Station
GSDP : Ground Safety Data Package
GSE : Ground-Support Equipment
GSE : Gyro Stellar Estimator (sometimes denoted GSTE)
GSFC : Goddard Space Flight Center
GSTP : General Support Technology Programme (ESA)
GT : Guaranteed Time
GUI : Graphical User Interface
GW : Gravitational Wave
GXB : Galactic (halo) X-ray Background
G_ee : single-sided power spectral density
H : Hyperboloid
H/K : HouseKeeping (data)
H/R : Height over Radius (of a disk)
H/W : Hardware
HASTR : High-Accuracy Star TRacker (sometimes denoted HA-STR)
HDF5 : Hierarchical Data Format version 5
HDLC : High-level Data Link Control (protocol)
HDRM : Hold Down and Release Mechanism
HDRMA : HDRM Actuator
HEAPA : High-Energy AstroPhysics Association (Japan)
HEO : High Earth Orbit
HEPA : High‐Efficiency Particulate Air (filter)
HEU : Hot and Energetic Universe
HEW : Half-Energy Width (refers to the Point Spread Function)
HEX : HEXapod
HF : High Frequency
HFSE : High-Frequency Spatial Error
HGA : High-Gain Antenna
HICP : Harmonised Index of Consumer Prices
HJ : Hot Jupiter
HK : HouseKeep(ing)
HKTM : HouseKeeping TeleMetry (data)
HN : HarNess
HP : High Precision neutron (model)
HP-STR : High-Performance Star TRacker
HPD : Half-Power spot Diameter
HPO : High-performance Pore Optics (synonym for a stack of mirror plates)
HR : High-Resolution (event)
HRDM : Misspelling of HDRM
HRMA : High-Resolution Mirror Assembly (Chandra)
HS : Heat Switch
HSC : High Speed Cutting
HSIA : Hardware-Software Interaction Analysis
HTR : HeaTeR
HU : Heat Up
HV-HPC : High-Voltage High-Power Command
HXI : Hard X-ray Imager (Hitomi)
HXT : Hard X-ray Telescope (Hitomi)
HZW : Half-Zero-Width
H_2O : Water
H_2O_2 : Peroxide
HiREFS : High-Resolution Erect Field Spectrometer
I : Italia
I-AR : Instrument(s) Acceptance Review
I-CDR : Instrument(s) Critical Design Review 
I-O : Inner-Outer (mirror stracks)
I-PRR : Instrument(s) Preliminary Requirements Review
I-QR : Instrument(s) Qualification Review
I/F : InterFace
I2C : Inter-Integrated Circuit (also denoted I^{2}C)
IAAT : Institut f\"{u}r Astronomie und Astrophysik - Universit\"{a}t T\"{u}bingen
IAC :  Intermediate Alignment Check
IACHEC : International Astronomical Consortium for High-Energy Calibration
IAP : (Czech) Institute of Atmospheric Physics
IAPS : Istituto di Astrofisica e Planetologia Spaziali
IAS : Institut d'Astrophysique Spatiale
IASF : Istituto di Astrofisica e Fisica dello Spazio
IBF : Ion-Beam Figuring
IC : Instrument Chamber
IC : Integrated Circuit
IC : Inter-Calibration
ICC : Instrument Consortia Consolidation
ICC : Instrument Control Centre
ICC : Internal Contamination Control
ICD : Interface Control Document
ICM : Intra-Cluster Medium
ICPU : Instrument Control and Power(-distribution) Unit
ICS : Inner Cold (Cryogenic) Shield (WFI)
ICS : Ion Composition Sub-system
ICU : Instrument Control Unit
IDL : Interactive Data Language
IDM : Integrated Design Model
IEC : International Electrical Commission
IEM : Instrument Electronics Module
IEM : Interplanetary Electron Model
IF : Integration Frame
IFC : InterFace Controller
IFCA : Instituto de F\{\i}sica de CAntabria (CSIC-UC)
IGM : Inter-Galactic Medium
IID : Interface Identification Document
IMD : IDL based software for modelling the optical properties of multilayer films
IMF : Initial-Mass Function
IMP : International Monitoring Platform
IMU : Inertia Measurement Unit
INAF : Istituto Nazionale di Astrofisica
INS : Isolated Neutron Star
INVAR : INVARiable, a particular Nickel–Iron alloy with 36% Nickel and 64% Iron (FeNi36 or 64FeNi)
IOC : In-Orbit Commissioning
IOCR : In-Orbit Commissioning Review
IP : In-Plane
IP : Intermediate Polar
IP : Intersection Plane
IPA : IsoPropyl Alcohol (cleaner)
IPC : Industrial Policy Committee (ESA)
IPCS : Inner Passive Cold (Cryogenic) Shield
IPM : Integrated Progress Meeting
IPRAM : Interplanetary and Planetary Radiation Model (for human spaceflight)
IPRR : Instrument(s) Preliminary Requirements Review (2018/2019)
IR : InfraRed
IR : Interim Review
IRAP : Institut de Recherche en Astrophysique et Plan\'{e}tologie
IRB : InfraRed Background
IRD : Interface Requirements Document (Definition)
IRS : Institut f\"{u}r Raumfahrtsysteme (Stuttgart)
IS : Inner Stack
ISC : Instrument Science (and data) Centre
ISCO : Innermost Stable Circular Orbit
ISEE-3 : International Sun/Earth Explorer 3 spacecraft
ISL : In-band Spurious Level
ISM : Instrument Switching (Selection) Mechanism
ISM : Iso-Static Mount
ISO : International Organisation for Standardisation (Geneva, Switzerland)
ISR : Intermediate Status Review
ISS : International Space Station
ISV : Independent Software Validation
ISVV : Independent Software Verification and Validation
IT : Instrument Team
ITAR : International Traffic In Arms Regulation (USA)
ITC : Instrument Team Centre
ITO : Interface Technical Officer
ITT : Invitation To Tender
ITU : International Telecommunications Union
IWF : Institut f\"{u}r Weltraumforschung (Graz, Austria)
IXO : International X-ray Observatory (obsolete)
IXPE : Imaging X-ray Polarimetry Explorer (NASA)
Isp : SPecific Impulse
IxI : Interface versus (times) Interface (diagram / matrix)
J : Japan
JASRI : Japan Synchrotron Radiation Research Institute
JAXA : Japanese Aerospace eXploration Agency
JD : Justification Document
JFY : Japanese Fiscal Year
JIRA : issue tracking product (not an acronym, but a truncation of Gojira, the Japanese name for Godzilla)
JT : Joule-Thomson (cooler)
JT2K : Joule-Thomson Two Kelvin (cooler)
JTAG : Joint Test Action Group (reprogramming interface)
K : Kelvin
KA : acceptance (test) factor
KDE : Knowledge Drift Error
KDP : Key Decision Point (NASA)
KEK : High-Energy Accelerator Research Organisation (Japan)
KH : Kelvin-Helmholtz
KIP : Key Inspection Point
KO : Kick Off
KOH : Potassium Hydroxide
KOM : Kick-Off Meeting
KP : Key Programme (Project)
KQ : qualification (test) factor
KRE : Knowledge Reproducibility Error
Ka : Kapton
Km : model (margin) factor
Kmp : margin-policy factor
Kp : project (margin) factor
L0 : Level-0 (data)
L1 : First Lagrange libration point of the Earth-Moon / Sun system
L1 : First Large mission of the ESA Cosmic Vision 2020 programme (assigned to JUICE)
L1 : Level-1 (data)
L2 : Level-2 (data)
L2 : Second Lagrange libration point of the Earth-Moon / Sun system
L2 : Second Large mission of the ESA Cosmic Vision 2020 programme (assigned to Athena)
L3 : Level-3 (data)
L3 : Third Large mission of the ESA Cosmic Vision 2020 programme (assigned to LISA)
LA : (WFI) Large Array (obsolete, now called LDA)
LAM : Laser Additive Manufacturing
LAT : Lot Acceptance Test
LB : LoBe
LBF : Long-Beam Facility
LC : Load Case
LC : inductor (L) and capacitor (C)
LCL : Latch Current Limiter
LD : (WFI) Large Detector (one quadrant of the WFI LDA)
LDA : (WFI) Large Detector Array
LDS : Laser Displacement Sensor
LEAF : Large European Acoustic Facility (ESTEC)
LEAS : List of ESA Approved Standards
LED : Light Emitting Diode
LEO : Low Earth Orbit
LEOP : Launch and Early-Orbit Phase
LET : Linear Energy Transfer
LFA : Lead Funding Agencies
LFSE : Low-Frequency Spatial Error
LG : Linear Gain
LGA : Low-Gain Antenna
LHEP : Low and High Energy Parameterised (model)
LHP : Loop Heat Pipe
LIGO : Laser Interferometer Gravitational-wave Observatory
LIS : Local Interstellar Spectrum
LISA : Laser Interferometer Space Antenna
LISN : Line Impedance Stabilisation Network
LKE : L.K. Engineering
LL : Launch Lock
LL : Limit Load
LLA : Launch-Lock Assembly
LLBL : Low-Latitude Boundary Layer
LLI : Long Lead Item
LLNL : Lawrence Livermore National Laboratory
LMCO : Lockheed Martin COrporation
LMJ : Laser Micro-(water) Jet (cutting)
LNA : Low-Noise Amplifier
LOS : Loss Of Signal
LOSF : Line-Of-Sight Frame
LOSTF : Line-Of-Sight Target Frame
LPA : Large Pixel Array
LPTC : Large Pulse-Tube Cooler
LR : Low-Resolution (event)
LRR : Launch Readiness Review
LS : Launch(er) Segment
LS : Lower Section (of FMS)
LSA : Launch Service Agreement
LSB : Least Significant Bit
LSD : Linear Spectral Density
LSF : Line-Spread Function
LSFE : Low Spatial-Frequency Error
LSM : Line-Scanning Mode
LT : Laser Tracker
LTD : Low-Temperature Detector (conference)
LTP : Long-Term Proposal (SPring-8)
LURE : Laboratoire d’Utilisation du Rayonnement \'{E}lectromagn\'{e}tique
LUT : Look-Up Table
LUTGEN : LUT GENerator
LV : Launch Vehicle
LVA : Launch-Vehicle Adapter
LVD : Low Voltage Directive
LVDS : Low-Voltage Differential Signalling
LVT : Lot Validation Testing
LW : Launch Window
LXB : Local (bubble) X-ray Background
L_bol : Bolometric Luminosity
L_edd : Eddington Luminosity
LoC : Lines of Code
LoR : Level of Resources (ESA speak for money)
LoS : Letter of Support
LoS: Line of Sight
M55J : Ultra-high modulus carbon fibre
MA : Mirror Assembly
MA : Mirror Axis
MACS : MAssive Cluster Survey
MAD : Mirror-Assembly Demonstrator
MAF : Mirror-Assembly Frame
MAIT : Manufacturing, Assembly, Integration, and Test
MAL : MALindi ground station (Kenya)
MAM : Mirror-Assembly Module
MAMD : Mirror Assembly Module Demonstrator
MAR : MARgin
MAR : Mission Adoption Review
MASD : Median Axial Slope Deviation
MAXI : Monitor of All-sky X-ray Image
MBD : Mission Budget Document
MC : Mirror Cover
MC : Monte Carlo
MCA : Mirror-Cover Assembly
MCD : Modified Condition and Decision
MCEM : Mirror-Cover Ejection Mechanism
MCF : Mirror Calibration Facility
MCI : Mass Centring Inertia
MCI : Mass, Centre-of-gravity, and Inertias
MCM : Multi-Chip Module
MCP : Mirror Calibration Plan
MCPL : Monte Carlo Particle List
MCR : Mission Consolidation Review (2016)
MCS : Mission-Control System
MCV : Magnetic Cataclysmic Variable
MD : Magnetic Diverter
MD : Mass Dummy
MD : Mirror Demonstrator
MDR : Model Development Review
MEA : Main Error Amplifier
MEKAL : Rolf Mewe, Jelle Kaastra, and Duane Liedahl
MEM : Meteoroid Engineering Model
MEUR : Million EURo
MFP : Mean Free Path
MFR : Mission Formulation Review (2019)
MGSE : Mechanical Ground-Support Equipment
MGU : Motor Gear Unit
MHD : Magneto-Hydro Dynamics
MHI : Mitsubishi Heavy Industries
MIABWG : Milano-IASF Athena Background Working Group
MICD : Mechanical Interface Control Drawing
MIL : United States of America Department of Defence Standard
MIM : Metal-Insulator-Metal
MIMO : Multiple-Input, Multiple-Output
MIP : Mandatory Inspection Point
MIP : Minimum Ionising Particle
MIP : Movable Instrument Platform (obsolete)
MIRD : Mission Implementation Requirements Document
MIST: Mechanical Integrated Services and Technologies (GSFC)
MJD : Modified Julian Date
MKE : Mean Knowledge Error
ML : Multi-Layer (coating)
MLA : Multi-Lateral Agreement
MLG : MaLarG\"ue ground station (35-m ESA ground station in Argentina)
MLI : Multi-Layer Insulation
MLT : Media Lario Technologies
MM : Mass Model
MM : Mirror Module
MMA : Movable Mirror Assembly
MMIC : Microwave Monolithic Integrated Circuit
MMPDS : Metallic Materials Properties Development and Standardisation
MMPP : Materials, Mechanical Parts, and Processes
MOA : Mirror Optical Axis
MOC : Mission Operations Centre (ESA)
MOC : Molecular Organic Contamination
MOL : Mock Observing List
MOOG : commercial motion and flow-control solutions provider (actually not an acronym, refers to Bill Moog who started the company)
MOP : Mock Observing Plan (Mock Observation Plan)
MOS : Metal-Oxide Semiconductor
MOS : Mock Observing Sequence
MOSFET : Metal–Oxide–Semiconductor Field-Effect Transistor
MOT : Mock Observing Timeline
MPCB : Materials, Mechanical Parts, and Processes Control Board
MPE : Max-Planck-Institut f\"{u}r Extraterrestrische Physik
MPE : Mean Performance Error
MPG : Max-Planck Gesellschaft
MPG/HLL :  Max-Planck-Gesellschaft/HalbLeiterLabor (semiconductor laboratory)
MPPT : Maximum Power Point Tracking
MPS : Max-Planck Society
MPS : Mission-Planning System
MPTC : Miniature Pulse-Tube Cooler
MPWG : Mission Performance Working Group
MR : Medium-Resolution (event)
MRD : Mission Requirements Document
MRR : Manufacturing Readiness Review
MRR : Mission Readiness Review
MS : (ESA) Member State
MS : MagnetoSheath
MS : MicroSoft
MS : Mirror Structure
MSB:  Most Significant Bit
MSBL : MagnetoSheath Boundary Layer
MSC/NASTRAN : Software for finite-element analysis
MSFC : Marshall Space Flight Center
MSPS : Mega Symbols per Second
MSR : Mission Selection Review
MSS : Mirror SunShield
MSSA : Mirror SunShield Assembly
MSSDM : Mirror SunShield Deployment Mechanism
MSU : Moscow State University
MT : Mission Time
MTBF : Mean Time Between Failure
MTH : Mechanical-THermal (testing)
MTL : Mission Time Line
MTTR : Mean Time To Repair
MUSD : Million US Dollar
MUX : MUltipleXing (factor)
MV : Multi-Variant
MW : Multi-Wavelength
MXS : Modulated X-ray Source
M^2 : Excess noise in X-IFU energy resolution
Mngt : Management
MoA : Memorandum of Agreement
MoI : Moment of Inertia
MoM : Minutes of Meeting
MoS : Margin of Safety
MoU : Memorandum of Understanding 
Ms : Mega-second
N : Nominal
N+R : Nominal plus Redundant
N/m : Newton per meter
NASA : National Aeronautics and Space Administration
NAST : NASA Athena Science Team
NC : Non-Compliant
NCA : Nickel-Cobalt-Aluminium
NCR : Non-Conformance Report
NCTS : Non-Conformance Tracking System
NDA : Non-Disclosure Agreement
NEA : Noise Equivalent Angle
NEA : Non-Explosive Actuator
NED : NASA/IPAC Extragalactic Database
NEP : Noise-Equivalent Power
NFI : Narrow-Field Imaging
NGRM : Next-Generation Radiation Monitor (ESA)
NHXM : New Hard X-ray Mission
NH_4OH : Ammonia Hydroxide
NID : Non-Ionising Dose
NIEL : Non-Ionising Energy Loss
NIR : Near Infra-Red (wavelength)
NIST : National Institute of Standards and Technology
NL : NetherLands
NLS : Narrow-Line Seyfert (galaxy)
NM : Neutron Monitor
NMOS : N-type Metal-Oxide Semiconductor
NNO : New NOrcia (35-m ESA ground station near Perth, Australia)
NOP : Nominal Operational Phase
NPSL : NASA Parts Selection List
NRB : Non-Conformance Review (board)
NS : Neutron Star
NSLS : National Synchrotron Light Source (Brookhaven National Laboratory)
NVR : Non-Volatile Residue
NXB : Non-X-ray (i.e., particle) Background
Nm : Newton meter
OAB : Osservatorio Astronomico di Brera
OAPA : Osservatorio Astronomico di PAlermo
OAR : Open Area Ratio
OAT : Off-Axis Test
OBC : On-Board Computer
OBCP : On-Board Control Procedure
OBDH : On-Board Data Handling
OBF : Optical Blocking Filter (WFI)
OBM : On-Board Metrology
OBMS : On-Board Metrology Sensor
OBMT : On-Board Metrology Target
OBSM : On-Board-Software Maintenance
OBSW : On-Board SoftWare
OBT : On-Board Time
OCM : Orbit-Control Manoeuvre
OCS : Outer Cold Shield (WFI)
OD : Overall Diameter
ODF : Observation Data File
ODI : Open Data Interface
OF : Obscuration Factor
OFDR : Optical Frequency-Domain Reflectometry
OFHC : Oxygen-Free High-conductivity Copper
OGS : Operational Ground Segment
OGSE : Operations Ground Support Equipment
OGSE : Optical Ground-Support Equipment
OH : Optical Head
OIRD : Operations Interface Requirements Document
OM : Optics Module
OMA : Opto-Mechanical Assembly
OMADA : Opto-Mechanical And Detector Assembly
OMC : On-Model Centroid
OMF : On-board Metrology Frame
OOF : Out Of Focus
OOP : Out-Of-Plane
OPD : Optical Path Difference
ORR : Operational Readiness Review
OS : Outer Stack
OSG : Observatory Science Goal
OT : Open Time
OTAC : Observation Time Allocation Committee
OTF : Optical Transfer Function
OTS : Off-The-Shelf (component)
OU : Open University
OUTER : Outer-radius SPO TDA
OV : Outer Vessel
ObAS : Observatoire Astronomique de Strasbourg
OoM : Order of Magnitude
P : Paraboloid
P : Portugal (sometimes erroneously used for Poland)
P-H : Paraboloid-Hyperboloid
P-S : Primary-Secondary (mirror stacks)
P/F : Pass/Fail
P/F : PlatForm
P/L : PayLoad
PA : Product Assurance
PAC : PArticulate Contamination
PAD : Parts Approval Document
PAL : Project Authorisation Letter (NASA)
PAMELA : Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics
PAN : Penetrating particle ANalyser
PANTER : PAntolsky Neuried TEstanlage Röntgen
PAP : Product Assurance Plan
PARD : Product Assurance (and safety) Requirements Document
PARS : Product Assurance Requirements Specification
PAU : Pattern Analyser Unit
PB : Pencil Beam
PBH : Primordial Black Hole
PC : Pre-Cooling (point)
PCA : Principal Component Analysis
PCB : Parts Control Board
PCB : Printed Circuit Board
PCDU : Power Conditioning and Distribution Unit
PCLA : Preliminary Coupled-Load Analysis
PCM : Power Conditioning Module
PCM : Pressurised Cargo Module
PCN : Process Change Notice
PCOS : Physics of the COSmos (NASA)
PCP : Programmatic and Cost (review) Panel
PCS : Physical Coding Sublayer
PCS : Primary Coordinate System
PCU : Pixel Correction Unit
PDD : Payload Definition/Description Document
PDE : Performance Drift Error
PDF : Portable Document Format (Adobe Acrobat)
PDHU : Payload Data-Handling Unit
PDR : Preliminary Design Review
PDU : Power Distribution Unit
PEC : Pointing-Error Contributor
PEE : Pointing-Error Engineering
PEEH : Pointing Error Engineering Handbook
PEEK : PolyEther Ether Ketone (plastic/polymer)
PEET : Pointing-Error Engineering Tool
PER : Process and Equipment Review
PES : Pointing-Error Source
PET : Photon-Event Table
PFM : Proto-Flight Model
PFO : Particle Fall-Out
PFU : Pixel Filter Unit
PGSE : Pressure Ground Support Equipment
PH : Pulse Height
PHA : Pulse Height Amplitude
PHU : Partial Heat Up
PI : Photon Ionisation
PI : Polyimide
PI : Principal Investigator
PI : Pulse Integral
PID : Process Identification Document
PID : Proportional Integral Derivative
PIF : Proton Irradiation Facility (at PSI)
PIND : Particle Impact Noise Detection
PIXE : Particle-Induced X-ray Emission
PL : PayLoad
PL : Physics List
PL : PoLand
PLA : PayLoad Adaptor
PLF : PLatForm
PLF : PayLoad Fairing
PLP : Pre-Launch Phase
PM : Progress Meeting
PM : Project Manager
PMM : Project Manager(s) Meeting
PMOS : P-type Metal-Oxide Semiconductor
PMP : Parts, Materials, and Processes
PN : Pseudo-Noise (ranging)
PO : Pressure Oscillation
POL : Point-Of-Load
POP : Post-Operations Phase
POPS : Post-OPerationS Phase
PPBE : Planning, Programming, Budgeting, and Execution (NASA)
PPR : Performance and Process Review
PPRR : Payload PRR
PPS : Pulse Per Second
PR : Preliminary Review
PR1 : Progress Review #1
PRD : (X-IFU) Performance Requirements Document (obsolete)
PRD : Project Reference Database
PRE : Performance Reproducibility Error
PRE : Programmable Real-time Emulator
PRECO : PRE-COmpound Model
PRESPO : PReparation of the Engineering model of the SPO optics (continuation of SPOHO)
PRR : Preliminary Requirements Review
PS : Plasma Sheet
PS : Primary Structure
PS : Project Scientist
PSBL : Plasma Sheet Boundary Layer
PSD : Power Spectral Density
PSF : Point Spread Function
PSI : Paul Scherrer Institute
PSM : Power Supply Module
PSO : Polished Silicon Optics
PSPC : Position Sensitive Proportional Counter
PSU : Penn State University
PSU : Power-Supply Unit
PT : Product Tree
PT : Pulse Tube
PTB : Physikalisch-Technische Bundesanstalt
PTC : Pulse-Tube Cooler
PTF : Palomar Transient Factory
PTFE : PolyTetraFluoroEthylene (Teflon)
PTR : Post Test Review
PU : Partner User (SPring-8)
PUS : Packet Utilisation Standard
PUS : Packet Utility Service(s)
PV : Performance Verification
PVP : Performance Verification Phase
PW : Plain Weave
PWN : Pulsar-Wind Nebula
PWS : Plain Weave Spread
P_c : confidence level
P_ee : linear spectral density
PaCSXI : Passive Cryogenic SIM/X-IFU Interface (working group)
Ph : Phase
PhAx : Phase-A eXtension
PoC : Point of Contact
ProdEx : PROgramme de D\’{e}veloppement d'Exp\’{e}riences scientifiques (ESA)
Q : Qualification (testing)
QA : Quality Assurance
QCI : Quality Conformance Inspection
QCM : Quartz Crystal Microbalance
QGS : Quark Gluon String (model)
QL : Qualification Load
QLA : Quick-Look Analysis
QM : Qualification Model
QML : Qualified Manufacturers List
QPL : Qualified Parts List
QR : Qualification Review
QRR : Qualification Review Report
QSL : Qualification Status List
QSL : Quasi-Static Load
R : Redundant
R : Resistor
R : Review (of design)
R1500 : Outer-radius mirror module
R200 : Radius at which the mean mass density exceeds the critical density by a factor of 200
R277 : Inner-radius mirror module
R500 : Radius at which the mean mass density exceeds the critical density by a factor of 500
R737 : Middle-radius mirror module
RACI : Responsible / Accountable / Consulted / Informed (matrix)
RADLAT : RADiation Lot Acceptance Test
RAL : Rutherford Appleton Laboratory
RAM : Random-Access Memory
RAMS : Reliability, Availability, Maintainability, and Safety
RAS : Rotating Anode Source
RB : Requirements Baseline
RCA : Radio Corporation of America (laboratories)
RCS : Reaction Control Sub-system
RD : Reference Document
RDM : Radiation Design Margin
RE : Radiated Emission
RE : Radiative Emissivity
REL : (SEPEM) Reference Event List
RERMM : Radiation Environment Research from Multiple Monitors
RF : Radio Frequency
RFC : Radio Frequency Compatibility
RFD : Request For Deviation
RFDU : Radio Frequency Distribution Unit
RFI : Request For Information
RFQ : Request For Quotation
RFW : Request For Waiver
RGMM : Reduced Geometrical Mathematical Model
RH : Relative Humidity
RHA : Radiation Hardness Assurance
RID : Review Item Discrepancy
RIKEN : Rikagaku Kenkyūsho (research institute in Japan)
RIU : Remote Interface Unit
RJ : Rotary Joint
RJF : Requirement Justification File
RJF : Requirements Justification File
RKE : Relative Knowledge Error
RMAP : Remote Memory Access Protocol
RMF : Redistribution (Response) Matrix File
RML : Recovered Mass Loss
RMU : Rate Measurement Unit
RMXP : Rotation-powered Millisecond X-ray Pulsar
RN : Recurrent Nova
RNG : RaNGing
RO : Reverse Osmosis
ROM : Read-Only Memory
ROM : Rough Order of Magnitude
RP : Random Process
RPB : RTU Power Board
RPE : Relative Performance Error
RPSD : Root Power Spectral Density
RR : Readiness Review
RS : Radiative Susceptibility
RS : Raster Scan
RS : Reed-Solomon
RS485 : Recommended Standard 485
RSP : ReSPonse (file)
RSS : Root Sum Square
RSU : Read-Shutter Unit
RT : Radiative Transfer
RT : Ray Tracing
RT : Remote Terminal
RT : Room Temperature
RTD : Reference Telescope Design
RTE : Real-time Emulator
RTG4 : Reprogrammable, radiation-tolerant FPGA from Microsemi
RTM : Resin Transfer Moulding
RTMM : Reduced Thermal Mathematical Model
RTN : ReTurN
RTPU : Remote Thermal and Power Unit
RTS : Rotating Target Source
RTU : Remote Terminal Unit
RV : Random Variable
RVT : Radiation Verification Testing
RW : Reaction Wheel
RWA : Reaction-Wheel Assembly
RWE : Reaction-Wheel Electronics
RXTE : Rossi X-ray Timing Explorer
R_n : normal Resistance
Rg : Gravitational (Schwarzschild) radius
Rsh : shunt Resistor
Rx : Rotation around the x-axis
Ry : Rotation around the y-axis
Rz : Rotation around the z-axis
S : Sweden
S-K : Sulphur K (line)
S/B : Signal to Background (ratio)
S/C : SpaceCraft (also denoted SC)
S/S : SpreadSheet
S/S : Sub-System
S3R : Sequential Switching Shunt Regulator
SA : Solar Array
SA : Sun Acquisition
SAA : Solar-Aspect Angle
SAC : Self Anti-Coincidence
SADE : Solar-Array Drive Electronics
SADM : Solar-Array Drive (Deployment) Mechanism
SALEX : SAcrificial Lateral EXtension
SAM : Self-Assembled Monolayer
SAOM : Solar-Array Orientation Mechanism
SAS : Sun-Acquisition Sensor
SAST : SKA-Athena Synergy Team
SB : Super Bubble
SBT : Service des Basses Temp\'{e}ratures
SC : SpaceCraft (also denoted S/C)
SC : Steering Committee
SC-1 : Standard Clean (step) 1 = APM = NH_4OH + H_2O_2 + H_2O
SCC : Stress Corrosion Cracking
SCCM : Standard Cubic Centimeters per Minute
SCF : Software Configuration File
SCI : SCIence Directorate (of ESA)
SCI : SCIence directorate (ESA)
SCIOBJ : SCIence OBJective
SCJ : Science Council of Japan
SCNTPL : \'{S}l\k{a}skie Centrum Naukowo - Technologiczne Przemys\l{}u Lotniczego (company in the field of advanced composites)
SCOM : SCience Operations Manager
SCRACS : SpaceCraft Reference Attitude Coordinate System
SCSB : Space Components Steering Board
SD : Single Degenerate
SD : Space Debris
SD : Stealth Dicing
SDC : Science Data Centre
SDD : Silicon Drift Detector
SDF : System Design File
SDMP : Space-Debris Management Plan
SDMP : Space-Debris Mitigation Plan
SDP : Science Demonstration Phase
SDPT : Science Data Processing Team
SDR : Software Design Review
SEB : Single-Event Burn-out
SEB : System Engineering Board
SEDR : Single-Event Dielectric Rupture
SEE : Single-Event Effect
SEFI : Single-Event Functional Interrupt
SEGR : Single-Event Gate Rupture
SEL : Single-Event Latch-up
SEL1 : Sun-Earth Lagrange point 1
SEL2 : Sun-Earth Lagrange point 2
SEP : Soft-Energy Proton
SEP : Solar Energetic Particle (event)
SEP : System Engineering Plan
SEPEM : Solar Energetic Particle Environment Modelling
SEPP : Science Exploitation and Preservation Platform
SES : Sun-Earth-Satellite (angle)
SES : Sun-Earth-Spacecraft (angle)
SET : Single-Event Transient
SEU : Single-Event Upset
SF : Source Follow (readout mode)
SFR : Star-Forming Region
SFT : Short Functional Test
SG : Science Goal
SGD : Soft Gamma-ray Detector (Hitomi)
SGICD : Space-to-Ground Interface Control Document
SGO : Slumped-Glass Optics
SGS : Science Ground Segment
SI : International System (of units)
SI : Single Issue
SIB : Science Instruments Bench ("core" of the SIM, for instance without radiators)
SICO : Science Impact of COst saving-options exercise (CORE exercise at X-IFU level)
SIM : Science Instruments Module
SIMO : Science Impact (assessment) of Mass-saving Options
SIMPOSiuM : SIMulation of Pore OpticS and Modeling
SIMPUT : SIMulation inPUT (for SIXTE)
SIP : Science Implementation Plan
SIP : Science Instruments Platform
SIRD : Science Implementation Requirements Document
SIRENA : Software IFCA for Reconstruction of EveNts for Athena X-IFU
SIS : Satellite Interface Simulator
SISO : Single-Input, Single-Output
SITU : Sci(ence)-op(eration)s technical Information Technology Unit (ESAC)
SIXTE : SImulation of X-ray TElescopes
SKA : Square-Kilometre Array
SLAC : Stanford Linear Accelerator Center (now known as SLAC National Accelerator Laboratory)
SM : Structural Model
SM : System Margin
SMAC : company in the field of shock and vibration control
SMBH : Super-Massive Black Hole
SMD : Study Management Document
SME : Small or Medium(-sized) Enterprise
SMM : Simplified Mathematical Model
SMM : Structural Mathematical Model
SMP : Science Management Plan
SMP : Study Management Plan (obsolete, superseded by Study Management Document)
SMU : Spacecraft Management Unit
SN : SuperNova
SNR : Signal-to-Noise Ratio
SNR : SuperNova Remnant
SNR : Surface Normal Rotation (monochromator)
SOAD : Science Operations Assumptions (Agreements) Document
SOC : Science Operations Centre (ESA)
SOCCI : Science Operations Configuration Control Infrastructure
SOCD : Science Operations Concept Document
SOHO : SOlar Heliospheric Observatory
SOLEIL : Source Optimis\’{e}e de Lumi\`{e}re d’\’{E}nergie Interm\’{e}diaire du LURE
SOM : Spacecraft Operations Manager
SOVT : System Operations Validation Test
SP : Soft Proton (from the Sun)
SP : Stopping Power
SPA : Slice Pattern Analyser
SPA : Small Pixel Array
SPC : Science Programme Committee (ESA)
SPE : Solar Particle Event
SPENVIS : SPace ENVIronment, effects, and education System
SPEX : SPEctral X-ray (and UV modelling and analysis tool)
SPF : Single-Point Failure
SPI : Star-Planet Interaction 
SPIRIT : Silicon Pore Inner RadIus Tender (Inner-radius SPO TDA)
SPL : Sound Pressure Level
SPL : Space Physics List
SPM : Science Products Module (WFI)
SPM : Soft-Proton Monitor
SPO : Silicon Pore Optics
SPOEQM : Silicon Pore Optics Engineering Qualification Model
SPOHO : SPO Half-energy-width Optimisation
SPORT : Silicon Pore Optics Ray Tracing
SPORT : Silicon Pore Optics Ray-tracing Tool
SPORT : Silicon-Pore-Optics Rugidisation (and environmental) Test(ing activity)
SPR : Software Problem Report
SPring-8 : Super Photon ring-8 GeV
SQR : Simulator Qualification Review
SQUID : Superconducting QUantum Interference Device
SR : Status Review
SR1 : Status Review number 1
SR2 : Status Review number 2
SRD : Spacecraft Requirements Document
SRD : System Requirements Document
SRE : Science and Robotic Exploration directorate (obsolete, now called SCI)
SREM : Standard Radiation Environment Monitor
SRF : Software Reuse File
SRG : Spectrum - Roentgen - Gamma
SRIM : Stopping and Range of Ions in Matter
SRON : Stichting RuimteOnderzoek Nederland (Netherlands Institute for Space Research)
SRP : Solar Radiation Pressure
SRR : System Requirements Review
SRS : Shock Response Spectrum
SS : Single Scattering
SS : Sub-System
SSAC : Space Science Advisory Committee (ESA)
SSC : Senior Survey Committee (for ESA's L-class missions)
SSC : Synchrotron Self Compton
SSCE : Sun-S/C-Earth (angle)
SSCE : Sun-SpaceCraft-Earth (angle)
SSD : Solid-State Detector
SSEWG : Solar-System Exploration Working Group (ESA)
SSMM : Solid-State Mass Memory
SSS : Super-Soft (X-ray) Source
SST : Solid-State Telescope
ST : Science Team
ST : Specification Tree
STA : STAbility
STAR : Space Technology and Astrophysics Research (Li\`ege)
STC : Single Threshold Processing (pulse detection)
STDBY : STanD BYe
STEP : Standard for the Exchange of Product Data
STFC : Science and Technology Facilities Council
STI : Supra-Thermal Ion
STM : Structural-Thermal Model
STOP : Structural, Thermal, and Optical Performance
STR : STRucture
STR : Star TRacker
STT : STar Tracker
STT : Science Topic Team
SUM : Software User Manual
SUPL : Space Users Physics List
SVM : SerVice Module
SVS : Spacecraft Venting Sub-system
SVTL : Safety Verification Tracking Log
SVVP : Software Validation and Verification Plan
SW : Solar Wind
SWCX : Solar Wind Charge eXchange
SWG : Science Working Group
SWP : Synergy White Paper
SWR : SpaceWire Router
SXI : Soft-X-ray Image (Hitomi)
SXRB : Soft X-Ray Background
SXS : Soft-X-ray Spectrometer (Hitomi)
SXT : Soft-X-ray Telescope (Hitomi)
SZ : Sunyaev-Zeldovitch (effect)
SciRD : Science Requirements Document
Si : Silicon
SiC : Silicon-Carbide
SoC : Status of Compliance
SoW : Statement of Work
SpW : Space Wire (bus protocol)
T : Test
T/O : Trade-Off
T0 : Launch
TAA : Technical Assistance Agreement
TAC : Time Allocation Committee
TAI : International Atomic Time
TAM : Transient Analysis Module (SPM/WFI)
TAS : Thales Alenia Space
TAS-F : Thales Alenia Space France
TAS-I : Thales Alenia Space Italy
TB : Thermal Balance
TBC : To Be Confirmed
TBD : To Be Defined
TBS : To Be Specified
TBW : To Be Written
TC : TeleCommand(ing)
TC : Thermal Cycling
TCB : Barycentric Coordinate Time
TCI : Technology Conformance Inspection
TCM : Trajectory (Transfer) Correction Manoeuvre
TCS : Thermal-Control Sub-system
TCU : Thermal Control Unit
TCWG : Telescope Calibration Working Group
TD : Telescope Demonstrator
TDA : Technology Demonstrator (Development) Activity
TDB : Barycentric Dynamical Time
TDM : Time-Domain Multiplexing
TDP : Technical Data Package
TDP : Technology Development Plan
TEB : Tender Evaluation Board
TEC : TEChnical Directorate (of ESA)
TEC-EES : space Environments and Effects Section in ESA's TEChnical Directorate
TECNET : TEChnology NETwork (ESA)
TED : Thermo-Elastic Distortion
TES : Transition Edge Sensor
TF : Thermal Filter (obsolete; use THF instead)
TF : Transfer Function
TFA : Thin Filter Assembly
TGT : Thermal Gradient Test
THESEUS : Transient High-Energy Sky and Early-Universe Surveyor 
THF : Thermal Filter (replaces TF)
TID : Total Ionising Dose
TIDL : Total Ionising Dose Level
TIDS : Total Ionising Dose Sensitivity
TIM : Technical Interface Meeting (ESA-NASA)
TLM : TeLeMetry
TM : Technology Management
TM : TeleMetry
TMD : Tuned (Tunable) Mass Damper
TML : Total Mass Loss
TMM : Thermal Mathematical Model
TMU : Thermal Management Unit
TN : Technical Note
TNID : Total Non-Ionising Dose
TNIDL : Total Non-Ionising Dose Level
TNIDS : Total Non-Ionising Dose Sensitivity
TNO : Technical NOte
TO : Technical Officer
TP : Technical Panel
TP : Technology Plan
TP : Topical Panel
TP : Transfer Phase
TPA : Telescope Performance Assessment
TPC : Topical Panel Chair
TRA : Technology Readiness Assessment
TRB : Test Review Board
TRD : Technical Requirements Document
TRD : Telescope Reference Design
TRD : Test Requirements Document
TRL : Technology Readiness Level
TRP : Temperature Reference Point
TRR : Test Readiness Review
TRS : Technical Requirement Specification
TRSL : Technology Readiness Status List
TRoPIC : single photon counting CCD camera, eROSITA prototype camera
TS : Technical Specification
TSM : Temperature Sensor Monitor
TT : Terrestrial Time
TT&C : Telemetry, Tracking, and Command(ing)
TTC : Telemetry and TeleCommand
TTV : Total Thickness Variation
TV : Thermal Vacuum
TV/TC : Thermal Vacuum/Thermal Cycling
TVAC : Thermal-VACuum (test / chamber)
TWG : Telescope Working Group
TWT : Traveling Wave Tube
TWTA : Traveling WaveTube Amplifier
TWTA : Travelling WaveTube Amplifier
Te2v : Teledyne e2v technologies
TiAl6V4 : Titanium-Aluminium alloy
ToF : Time of Flight
ToO : Target of Opportunity
Tx : Translation along the x-axis
Ty : Translation along the y-axis
Tz : Translation along the z-axis
UC : Use Case
UFO : Ultra-Fast Outflow
UG : University of Geneva
UK : United Kingdom
UKSA : United Kingdom Space Agency
UL : University of Leicester
ULIRG : Ultra-Luminous InfraRed Galaxy
ULPA : Ultra-Low-Particle Air (filter)
ULX : Ultra-Luminous X-ray (source)
UM : User Manual
URD : User Requirements Document
US : United States (of America)
US : Upper Section (of FMS)
USD : United States Dollar
UTC : Universal Coordinated Time
UV : Ultra-Violet
UVOB : Ultra-Violet Optical Bench
UniPa : University of Palermo
V : Volt
V&V : Verification and Validation
V/I : Voltage and/or current
VBQC : Vacuum Balance Quartz Crystal
VCB : Verification Control Board
VCD : Verification Control Document
VCDM : Verification Control Document and Matrix
VCM : Verification Control Matrix
VCM : Volatile Condensable Material
VDD : Drain-to-Drain Voltage
VEM : Viscous-Elastic Material
VERITAS : VErsatile Readout based on Integrated Trapezoidal Analogue Shapers
VERT-X : VERTical facility for X-ray testing (OAB)
VF : Vignetting Function
VF : Vignetting Function (Vignetting Factor)
VG : ViewGraph
VIS : VISible (wavelength)
VIS : Vibration Isolation Sub-system
VLM : Very Low-Mass (star)
VM : Verification Matrix
VO : Virtual Observatory
VRR : Verification Readiness Review
VSUB : Back-gate bias
VTT : Valtion Teknillinen Tutkimuskeskus (Finland's State Technical Research Centre)
W : Watt
W : Window (mode)
W-I : Wolter telescope of type I
W-ISC : X-IFU Instrument Science (and data) Centre
W-S : Wolter-Schwarzschild
W1 : Wolter-1 (design)
WA : Wolter(-1) Athena
WBS : Work Breakdown Structure
WCA : Worst-Case Analysis
WDA : WFI Data Analysis (software)
WDE : Wheel Drive Electronics
WEDM : Wire(-cut) Electrostatic Discharge Machining
WFEE : Warm Front-End Electronics
WFI : Wide-Field Imager
WFOIS : Wide-Field Optical/Infrared Surveyors
WFR : Weibull Failure Rate
WG : Working Group
WGS : World Geodetic System
WHIM : Warm-Hot Inter-galactic Medium
WISC : WFI Instrument Science (and data) Centre
WM : Windowed Mean
WMS : Windowed Mean Stability
WOLTER : Mid-radius SPO TDA
WP : White Paper
WP : Work Package
WPD : Work Package Description
WPM : Work Package Manager
WPR : Weighted Particle Rate
WPS : Work Package Structure
WSB : Weak Stability Boundary
WSS : Warm Support Structure
WST : WFI Science Team
X-COP : XMM-Newton Cluster Outskirts Project
X-IFU : X-ray Integral-Field Unit
X-ISC : X-IFU Instrument and Science (data) Centre
XARM : X-ray Astronomy Recovery (Replacement) Mission (“Hitomi-2”); obsolete and now called XRISM
XCM : X-IFU Consortium Meeting
XCMT : X-IFU Consortium Management Team
XCaT : X-IFU Calibration Team
XCoB : X-IFU Consortium Board
XDA : X-IFU Data Analysis (software)
XDIN : X-ray Dim Isolated Neutron (star)
XDIN : X-ray Dim Isolated Neutron star
XDS : X-ray Detector System
XEUS : X-ray Evolving-Universe Spectroscopy (obsolete)
XIFU : X-ray Integral-Field Unit
XISC : X-IFU Instrument Science Centre
XISC : X-IFU Science and data Centre
XMM : X-ray Multi-Mirror telescope
XMS : X-ray Micro-calorimeter Spectrometer (obsolete)
XOAT : X-ray Optical Assembly Tool (stacking robot)
XOU : X-ray Optical Unit
XPBF : X-ray Pencil-Beam Facility (at PTB)
XRB : X-Ray Binary
XRCF : X-Ray and Cryogenic Facility (at MSFC; formerly known as X-Ray Calibration Facility)
XRISM : X-Ray Imaging and Spectroscopy Mission; formerly known as XARM or "Hitomi-2"
XRR : X-ray Reflectometry
XRS : X-Ray Scattering
XRTG4 : X-Ray Tracer based on Geant-4
XSAT : X-IFU Science Advisory Team
XSS : X-ray Source System
XTA : X-ray Tube Assembly
YSH-70A : Ultra-high modulus composite
YSO : Young Stellar Object
aLIGO : Advanced LIGO
aVirgo : Advanced Virgo
au : Astronomical Unit
cf : confer / conferatur (compare with)
dE : delta Energy (energy resolution)
dMCR : delta Mission Consolidation Review (2017)
e2ePA : end-to-end Performance Assessment
eCaC : estimated Cost at Completion
eNCTS : ECLIPSE Non-Conformance Tracking System
eg : exempli gratia (for example)
f2f : face-to-face (meeting)
ie : id est (in other words)
kSZ : kinetic Sunyaev-Zeldovitch (effect)
ks : kilo-second
mgmt : ManaGeMenT
ppm: part per million
rtHz : square root of Hertz
sic : sic erat scriptum (thus was it written)
tSZ : thermal Sunyaev-Zeldovitch (effect)
uTCA : micro Telecommunications Computing Architecture (connector)
uVib : micro Vibrations
um : micrometer
z : redshift
µV : micro-Vibration