Athena acronym list, actively maintained by Jos de Bruijne (with input from Matthias Ehle). Additions, corrections, and feedback very welcome. Last updated 7 February 2019.
 
1g : Earth's gravity
3E : Electrical, Electronic, and Electro-mechanical, sometimes referred to as triple E (components)
A : Abell
A : Acceptance (testing)
A64 : Ariane-6.4 (launcher)
AC : Alternating Current
AC : Anti-Coincidence
ACD : Anti-Coincidence Detector
ACE : Advanced Composition Explorer (spacecraft)
ACO : Athena Community Office
ACSS : Athena Common Software System
ACSST : Athena Common Software System Team
AD : Adjusted Derivative (pulse detection)
AD : Analogue to Digital
ADA : Athena Data Analysis
ADC : Analogue-to-Digital Converter
ADR : Adiabatic Demagnetisation Refrigerator
ADS : Airbus Defence and Space
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
AIB : Analogue Interface Board
AIT : Assembly, Integration, and Test
AIV : Assembly, Integration, and Verification
AKE : Absolute Knowledge Error
ALAT : Air Liquide Advanced Technologies
ALD : Atomic Layer Deposition
AMA : Athena Mirror Assembly
AMXP : Accretion-powered Millisecond X-ray Pulsar
AO : Announcement of Opportunity
AOB : All Other Business
AP : As Proposed
APC : AstroParticule & Cosmologie (Paris)
APE : Absolute Pointing Error
APEC : Astrophysical Plasma Emission Code
APM : Ammonia Peroxide Mix
APS : Active Phase Shift
APS : Active-Pixel Sensor
AR : (pixel) Aspect Ratio
AR : Acceptance Review
AREMBES : Athena Radiation Environment Models and X-Ray Background Effects Simulators (Athena Radiation Environment Modelling Background EStimator)
ARF : Ancillary Response File
ARSE : Angular Resolution Sensitivity Exercise
ASA : Athena Science Archive
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
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
ASW : Application Software
ATAC : Athena Time Allocation Committee
ATHENA : Advanced Telescope for High-ENergy Astrophysics
ATP : Authorisation To Proceed
AVM : AVionics Model
AWG : Astronomy Working Group (ESA)
Al : Aluminium
ApC : Aperture Cylinder
B4C : Boron_4-Carbide
BAM : Background Analysis Module (WFI)
BAR : Background Assessment Report
BB : Bread Board
BBFB : BaseBand FeedBack
BCB : BenzoCycloButene (coating)
BCG : Brightest Cluster Galaxy
BE : Background Estimate
BEAR : Bending magnet for Emission, Absorption, and Reflectivity (beam line)
BEE : Back-End Electronics
BESSY : Berliner ElektronenSpeicherring-gesellschaft f\"{u}r SYnchrotronstrahlung
BEUR : Billion EURo
BEaTriX : Beam Expander Testing X-ray facility
BF : Blocking Factor
BGD : BackGrounD
BH* : Stellar-mass Black Hole
BHB : Black-Hole Binary 
BHC : Black-Hole Candidate
BKG : BacKGround
BL : Boundary Layer
BND : Beam-Normalisation Detector
BSC : Back-SCattering
BSE : Back-Scattered Electron
BUSD : Billion US Dollar
BoGEMMS : Bologna GEant4 Multi-Mission Simulator
Br : Bracket (spectral line)
C : Carbon
C/B : Clamp Band
CA : Conical(-approximation) Athena
CAD : Computer-Aided Design
CAS : Calibration ASsembly (X-IFU)
CAS : Czech Academy of Sciences
CBE : Current Best Estimate
CBK : Centrum Bada\'{n} Kosmicznych
CC : Cold Core (galaxy cluster)
CC : Cooling Chain
CC : Core Collapse (supernova)
CC : Cost-Constrained (mission)
CCCM : Channel-Cut Crystal Monochromator
CCDR : Closed-Cycle Dilution Refrigerator
CCM : Centre for Concepts in Mechatronics
CCN : Change-of-Contract Notice
CCSDS : Consultative Committee for Space Data Systems
CCU : Cryo-chain Control Unit
CD : Cool Down
CDE : Cooler Drive Electronics
CDF : Concurrent Design Facility
CDFS : Chandra Deep Field South
CDM  : Code-Domain Multiplexing (Code-Division Multiplexing)
CDMU : Central Data Management Unit
CDR : Critical Design Review
CE : Conductive Emissivity
CEA : Commissariat à l'\'{E}nergie Atomique
CF : Crest Factor
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)
CIL : Critical Items List
CIRSOS : Collaborative Iterative Radiation Shielding Optimisation System
CLS : Canadian Light Source
CM : Consortium Meeting
CM : Ministerial Council (meeting)
CMM : Coordinate Measuring Machine
CMZ : Central Molecular Zone
CN : Classical Nova
CNC : Computer Numerical Controlled
CND : CircumNuclear Disk
CNES : Centre National d'\'{E}tudes Spatiales
CORE : Cost-Oriented Reprogramming Exercise (Cost-driven Observation Reprogramming Exercise)
COSPAR : Committee On SPAce Research
CP : Calibration Plan
CPS : Central Plasma Sheet
CPS : Counts Per Second
CR : Cosmic Ray
CR : Count Rate
CRD : Calibration Requirements Document
CS : Conductive Susceptibility
CS: Cooling System
CSL : Centre Spatial de Li\`ege
CSS : Core Science Survey
CT : Compton Thick
CTA : Cherenkov Telescope Array
CTE : Coefficient of Thermal Expansion
CTP : Core Technology Programme
CTQ : Critical To Quality
CV : Cosmic Vision (programme of ESA's science directorate)
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
CoDR : Conceptual Design Review
ConOps : Concept of Operations
DAQ : Data AQuisition
DBRR : DCS Breadboarding for Risk Reduction
DCCM : Document Control and Change Manager
DCM : Double-Crystal Monochromator
DCS : Demonstrator of the (X-IFU) Cooling System
DCS : Detector Cooling System
DD : Double Degenerate
DDF : Design Definition File
DDT : Director’s Discretionary Time
DE : Detector Electronics
DEE : DE-Excitation Model
DEPFET : DEpleted P-channel Field-Effect Transistor
DF : De-Focusing
DJF : Design Justification File
DLL : Design Launch Load
DLR : Deutsches Zentrum für Luft- und Raumfahrt
DM : Demonstration Model
DMM : Design Maturity Margin
DMSO : DiMethyl SulfurOxide
DN : Dwarf Nova
DOORS : Dynamic Object Oriented Requirements System
DR : DRain current (readout mode)
DRE : Digital Read-out Electronics
DRL : Deliverables Requirement List
DRL : Document Requirements List
DT : Dead Time
DT : Discretionary Time
DTU : DTU Space (formally: National Space Institute, Danish: Institut for Rumforskning og Rumteknologi), part of the Technical University of Denmark
DoF : Degree of Freedom
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
ECAP : Erlangen Centre for Astroparticle Physics
EDM : Electrical Discharge Machining
EFM : Electrical Functions Model
EGSE : Electrical Ground-Support Equipment
EIA : Electronic Industries Alliance
EID : Experiment Interface Document
EIPS : Electron Impact Point Source
ELP : Low-Energy Proton
EM : Electrical Model
EM : Emission Measure
EM : Engineering Model
EMC : Electro-Magnetic Compatibility
EMI : Electro-Magnetic Interference
ENC : Equivalent Noise Charge
EP : Event Processing
EPIC : Energetic Particles and Ion Composition
EPIC : European Photon Imaging Camera
EPS : Electrical Power System
EQM : Electrical Qualification Model
ERB : Energy-Resolution Budget
ERT : Earth Reception Time
ES : Energy Scale
ESA : European Space Agency
ESAC : European Space Astronomy Centre (Villafranca, Spain)
ESC : ElectroStatiC
ESD : Electro-Static Discharge
ESHIEM : Energetic Solar Heavy Ion Environment Modelling
ESOC : European Spacecraft Operations Centre
ESTEC : European Space research and TEchnology Centre
ESU : Energy Storage Unit
ET : Einstein Telescope
EUV : Extreme UltraViolet
EXACRAD : Experimental Evaluation of Athena Charged Particle Background from Secondary Radiation and Scattering in Optics
EoR : Epoch of Reionisation 
F2F : Face-to-Face
FAR : Flight Acceptance Review
FC : (WFI) Fast Chip (obsolete, now called FD)
FCM : Four Crystal Monochromator
FD : (WFI) Fast Detector
FDM : Frequency-Domain Multiplexing
FE : Finite Element
FEA : Finite-Element Analysis
FEE : Front-End Electronics
FEM : Finite-Element Model
FF : Full Frame
FFT : Fast Fourier Transform
FINREF : FINancial REForm (framework for costing of ESA activities)
FIR : Facility Implementation Review
FL : Focal Length
FM : Flight Model
FMS : Fixed Metering Structure
FPA : Focal-Plane Assembly
FPC : Flow-Proportional Counter
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)
FR : Final Review
FR-II : Faranoff-Riley (type-2) radio galaxy
FRM : Fringe Reflection Metrology
FRT : Fringe-Reflection Technique
FS : Flight Spare (Model)
FW : Filter Wheel
FWA : Filter-Wheel Assembly
FY : Fiscal Year
FeKa : Iron K_\alpha (line)
FoM : Figure of Merit
FoR : Field of Regard
Lor : Level of Resources (ESA speak for money)
FuMo : Functional Model
G4 : GEANT4
GBH : Galactic Black Hole
GCR : Galactic Cosmic Ray
GDML : Geometry Description Markup Language
GEANT : GEometry ANd Tracking
GEANT4 : GEometry ANd Tracking version 4
GM : Gifford-McMahon (cooler)
GO : Guest Observer
GPS : Global Positioning System
GRAS : GEANT4 Radiation Analysis for Space
GRB : Gamma-Ray Burst
GRP : Giant Radio Pulse
GS : Ground Segment
GS : Ground Station
GSE : Ground-Support Equipment
GSFC : Goddard Space Flight Center
GSTP : General Support Technology Programme
GT : Guaranteed Time
GUI : Graphical User Interface
GW : Gravitational Wave
H/R : Height over Radius (of a disk)
HDLC : High-level Data Link Control (protocol)
HDRM : Hold Down and Release Mechanism
HEAPA : High-Energy AstroPhysics Association (Japan)
HEO : High-Earth Orbit
HEU : Hot and Energetic Universe
HEW : Half-Energy Width (refers to the Point Spread Function)
HGA : High-Gain Antenna
HICP : Harmonised Index of Consumer Prices
HPD : Half-Power spot Diameter
HR : High-Resolution (event)
HRDM : Misspelling of HDRM
HRMA : High-Resolution Mirror Assembly (Chandra)
HS : Heat Switch
HU : Heat Up
HXI : Hard X-ray Imager (Hitomi)
HXT : Hard X-ray Telescope (Hitomi)
H_2O : Water
H_2O_2 : Peroxide
HiREFS : High-Resolution Erect Field Spectrometer
I-PRR : Instrument(s) Preliminary Requirements Review
I/F : InterFace
I2C : Inter-Integrated Circuit (also denoted I^2C)
IAAT : Institut f\"{u}r Astronomie und Astrophysik - Universit\"{a}t T\"{u}bingen
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
IBF : Ion-Beam Figuring
IC : Instrument Chamber
ICC : Instrument Consortia Consolidation
ICC : Instrument Control Centre
ICD : Interface Control Document
ICM : Intra-Cluster Medium
ICPU : Instrument Control and Power-distribution Unit
ICS : Inner Cold Shield (WFI)
ICS : Ion Composition Sub-system
ICU : Instrument Control Unit
IEM : Interplanetary Electron Model
IFCA : Instituto de F\{\i}sica de CAntabria (CSIC-UC)
IGM : Inter-Galactic Medium
IID : Interface Identification Document
IMF : Initial-Mass Function
IMP : International Monitoring Platform
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)
IP : Intermediate Polar
IPC : Industrial Policy Committee
IPM : Integrated Progress Meeting
IPRAM : Interplanetary and Planetary Radiation Model (for human spaceflight)
IPRR : Instrument(s) Preliminary Requirements Review (2018/2019)
IR : InfraRed
IRAP : Institut de Recherche en Astrophysique et Plan\'{e}tologie
IRB : Infra-Red Background
IRD : Interface Requirements Document
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
IT : Instrument Team
ITC : Instrument Team Centre
ITT : Invitation To Tender
IXO : International X-ray Observatory (obsolete)
IXPE : Imaging X-ray Polarimetry Explorer (NASA)
JASRI : Japan Synchrotron Radiation Research Institute
JAXA : Japan Aerospace Exploration Agency
JFY : Japanese Fiscal Year
JT : Joule-Thomson
JT2K : Joule-Thomson Two Kelvin (cooler)
KEK : High-Energy Accelerator Research Organisation (Japan)
KH : Kelvin-Helmholtz
KOM : Kick-Off Meeting
KP : Key Programme (Project)
Ka : Kapton
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)
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 : Third Large mission of the ESA Cosmic Vision 2020 programme (assigned to LISA)
LA : (WFI) Large Array (obsolete, now called LDA)
LB : LoBe
LBF : Long-Beam Facility
LC : inductor (L) and capacitor (C)
LD : (WFI) Large Detector (one quadrant of the WFI LDA)
LDA : (WFI) Large Detector Array
LDS : Laser Displacement Sensor
LEAF : Large European Acoustic Facility
LEO : Low-Earth Orbit
LFA : Lead Funding Agencies
LG : Linear Gain
LGA : Low-Gain Antenna
LHP : Loop Heat Pipe
LIGO : Laser Interferometer Gravitational-wave Observatory
LISA : Laser Interferometer Space Antenna
LLBL : Low-Latitude Boundary Layer
LLI : Long Lead Item
LLNL : Lawrence Livermore National Laboratory
LMCO : Lockheed Martin COrporation
LNA : Low-Noise Amplifier
LPA : Large Pixel Array
LPTC : Large Pulse-Tube Cooler
LR : Low-Resolution (event)
LSF : Line-Spread Function
LT : Laser Tracker
LTD : Low-Temperature Detector (conference)
LTP : Long-Term Proposal (SPring-8)
LURE : Laboratoire d’Utilisation du Rayonnement \'{E}lectromagn\'{e}tique
LVA : Launch-Vehicle Adapter
L_bol : Bolometric Luminosity
L_edd : Eddington Luminosity
LoS : Letter of Support
MA : Mirror Assembly
MACS : MAssive Cluster Survey
MAD : Mirror-Assembly Demonstrator
MAIT : Manufacturing, Assembly, Integration, and Test
MAM : Mirror-Assembly Module
MAR : Mission Adoption Review
MASD : Median Axial Slope Deviation
MBD : Mission Budget Document
MCP : Mirror Calibration Plan
MCR : Mission Consolidation Review (2016)
MCV : Magnetic Cataclysmic Variable
MD : Magnetic Diverter
MDR : Model Development Review
MEKAL : Rolf Mewe, Jelle Kaastra, and Duane Liedahl
MEUR : Million EURo
MFP : Mean Free Path
MFR : Mission Formulation Review (2019)
MGSE : Mechanical Ground-Support Equipment
MIP : Minimum Ionising Particle
MIP : Movable Instrument Platform (obsolete)
MIRD : Mission Implementation Requirements Document
ML : Multi-Layer (coating)
MLA : Multi-Lateral Agreement
MLT : Media Lario Technologies
MM : Mass Model
MM : Mirror Module
MMA : Movable Mirror Assembly
MOC : Mission Operations Centre (ESA)
MOL : Mock Observing List
MOP : Mock Observing Plan (Mock Observation Plan)
MOS : Mock Observing Sequence
MOSFET : Metal–Oxide–Semiconductor Field-Effect Transistor
MOT : Mock Observing Timeline
MPE : Max-Planck-Institut f\"{u}r Extraterrestrische Physik
MPG : Max-Planck Gesellschaft
MPS : Max-Planck Society
MPTC : Miniature Pulse-Tube Cooler
MPWG : Mission Performance Working Group
MR : Medium-Resolution (event)
MRD : Mission Requirements Document
MRR : Manufacturing Readiness Review
MS : (ESA) Member State
MS : MagnetoSheath
MS : MicroSoft
MS : Mirror Structure
MSBL : MagnetoSheath Boundary Layer
MSFC : Marshall Space Flight Center
MSR : Mission Selection Review
MSU : Moscow State University
MUSD : Million US Dollar
MXS : Modulated X-ray Source
M^2 : Excess noise in X-IFU energy resolution
MoM : Minutes of Meeting
MoS : Margin of Safety
MoU : Memorandum of Understanding 
Ms : Mega-second
N : Nominal
NASA : National Aeronautics and Space Administration
NDA : Non-Disclosure Agreement
NED : NASA/IPAC Extragalactic Database
NEP : Noise-Equivalent Power
NFI : Narrow-Field Imaging
NGRM : Next-Generation Radiation Monitor
NHXM : New Hard X-ray Mission
NH_4OH : Ammonia Hydroxide
NID : Non-Ionising Dose
ODI : Open Data Interface
PIXE : Particle-Induced X-ray Emission
QR : Qualification Review
RERMM : Radiation Environment Research from Multiple Monitors
NIEL : Non-Ionising Energy Loss
NIST : National Institute of Standards and Technology
NLS : Narrow-Line Seyfert (galaxy)
NM : Neutron Monitor
NOP : Nominal Operational Phase
NS : Neutron Star
NSLS : National Synchrotron Light Source (Brookhaven National Laboratory)
NXB : Non-X-ray Background
OAB : Osservatorio Astronomico di Brera
OAPA : Osservatorio Astronomico di PAlermo
OBCP : On-Board Control Procedure
OBDH : On-Board Data Handling
OBF : Optical Blocking Filter (WFI)
OBSW : On-Board SoftWare
OBT : On-Board Time
OCS : Outer Cold Shield (WFI)
ODF : Observation Data File
OFDR : Optical Frequency-Domain Reflectometry
OGS : Operational Ground Segment
OGSE : Optical Ground-Support Equipment
OM : Optics Module
OMC : On-Model Centroid
OSG : Observatory Science Goal
OT : Open Time
OTAC : Observation Time Allocation Committee
OU : Open University
OUTER : Outer-radius SPO TDA
ObAS : Observatoire Astronomique de Strasbourg
OoM : Order of Magnitude
P-H : Paraboloid-Hyperboloid
P/L : PayLoad
PA : Product Assurance
PAMELA : Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics
PANTER : PAntolsky Neuried TEstanlage Röntgen
PAP : Product Assurance Plan
PB : Pencil Beam
PBH : Primordial Black Hole
PCA : Principal Component Analysis
PCOS : Physics of the COSmos (NASA)
PDD : Payload Definition/Description Document
PDR : Preliminary Design Review
PDU : Power Distribtuion Unit
PER : Process and Equipment Review
PET : Photon-Event Table
PFM : Proto-Flight Model
PH : Pulse Height
PHA : Pulse Height Amplitude
PHU : Partial Heat Up
PI : Photon Ionisation 
PIF : Proton Irradiation Facility (at PSI)
PL : PayLoad
PL : Physics List
PM : Progress Meeting
PPR : Performance and Process Review
PPRR : Payload PRR
PR1 : Progress Review #1
PRECO : PRE-COmpound Model
PRESPO : PReparation the Engineering model of the SPO optics (continuation of SPOHO)
PRR : Preliminary Requirements Review
PS : Plasma Sheet
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
PT : Product Tree
PT : Pulse Tube
PTB : Physikalisch-Technische Bundesanstalt
PTC : Pulse-Tube Cooler
PTF : Palomar Transient Factory
PU : Partner User (SPring-8)
PUS : Packet Utility Service(s)
PV : Performance Verification
PWN : Pulsar-Wind Nebula
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
QGS : Quark Gluon String (model)
FTF : FriTioF (model)
LHEP : Low and High Energy Parameterised (model)
BIC : BInary Cascade (model)
BERT : BERTini cascade (model)
HP : High Precision neutron (model)
EMV(X) : Variation of Standard EM package
QLA : Quick-Look Analysis
QM : Qualification Model
QR : Qualification Review
QSL : Quasi-Static Load
R : Redundant
R200 : Radius at which the mean mass density exceeds the critical density by a factor of 200
R500 : Radius at which the mean mass density exceeds the critical density by a factor of 500
RAL : Rutherford Appleton Laboratory
RAS : Rotating Anode Source
RCA : Radio Corporation of America (laboratories)
RCS : Reaction Control System
RE : Radiative Emissivity
REL : (SEPEM) Reference Event List
RFI : Request For Information
RIKEN : Rikagaku Kenkyūsho (research institute in Japan)
RMF : Redistribution (Response) Matrix File
RMXP : Rotation-powered Millisecond X-ray Pulsar
RN : Recurrent Nova
ROM : Rough, Order-of-Magnitude
RS : Radiative Susceptibility
RS485 : Recommended Standard 485
RT : Radiative Transfer
RT : Ray Tracing
RTS : Rotating Target Source
RTU : Remote Terminal Unit
RW : Reaction Wheel
RXTE : Rossi X-ray Timing Explorer
R_n : normal Resistance
Rg : Gravitational (Schwarzschild) radius
Rsh : shunt Resistor
S-K : Sulphur K (line)
S/C : SpaceCraft (also denoted SC)
S/S : SpreadSheet
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 1 = APM = NH_4OH + H_2O_2 + H_2O
SCCM : Standard Cubic Centimeters per Minute
SCI : SCIence directorate (ESA)
SCIOBJ : SCIence OBJective
SD : Single Degenerate
SD : Space Debris
SDC : Science Data Centre
SDD : Silicon Drift Detector
SDF : System Design File
SEP : Solar Energetic Particle
SEPEM : Solar Energetic Particle Environment Modelling
SOHO : SOlar Heliospheric Observatory
SPE : Solar Particle Event
SREM : Standard Radiation Environment Monitor
SUM : Software User Manual
TID : Total Ionising Dose
SDMP : Space-Debris Mitigation Plan
SEB : System Engineering Board
SEL1 : Sun-Earth Lagrange point 1
SEL2 : Sun-Earth Lagrange point 2
SEP : Soft-Energy Proton
SEP : Solar Energetic Particle (event)
SEPEM : Solar Energetic Particle Environment Modelling
SF : Source Follow (readout mode)
SG : Science Goal
SGD : Soft Gamma-ray Detector (Hitomi)
SGO : Slumped-Glass Optics
SGS : Science Ground Segment
SIB : Science Instruments Bench
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 : SIMulations of Pore Optics in SIlicon and Modelling
SIMPUT : SIMulation inPUT (for SIXTE)
SIP : Science Instruments Platform
SIRD : Science Implementation Requirements Document
SIRENA : Software IFCA for Reconstruction of EveNts for Athena X-IFU
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)
SMBH : Super-Massive Black Hole
SMD : Study Management Document
SME : Small or Medium(-sized) Enterprise
SMP : Science Management Plan
SMP : Study Management Plan (obsolete, superseded by Study Management Document)
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)
SOLEIL : Source Optimis\’{e}e de Lumi\`{e}re d’\’{E}nergie Interm\’{e}diaire du LURE
SP : Soft Proton (from the Sun)
SP : Stopping Power
SPA : Small Pixel Array
SPC : Science Programme Committee (ESA)
SPENVIS : SPace ENVIronment, effects, and education System
SPEX : SPEctral X-ray (and UV modelling and analysis tool)
SPI : Star-Planet Interaction 
SPIRIT : Inner-radius SPO TDA
SPL : Space Physics List
SPM : Science Products Module (WFI)
SPO : Silicon Pore Optics
SPOHO : SPO Half-energy-width Optimisation
SPORT : Silicon-Pore-Optics at Room Temperature
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
SRE : Science and Robotic Exploration directorate (obsolete, now called SCI)
SRG : Spectrum - Roentgen - Gamma
SRON : Stichting RuimteOnderzoek Nederland (Netherlands Institute for Space Research)
SRR : System Requirements Review
SRS : Shock Response Spectrum
SS : Single Scattering
SSAC : Space Science Advisory Committee
SSC : Senior Survey Committee (for ESA's L-class missions)
SSC : Synchrotron Self Compton
SSD : Solid-State Detector
SSEWG : Solar-System Exploration Working Group (ESA)
SSS : Super-Soft (X-ray) Source
SST : Solid-State Telescope
STC : Single Threshold Processing (pulse detection)
STDBY : STanD BYe
STFC : Science and Technology Facilities Council
STM : Structural-Thermal Model
STR : Star TRacker
STT : Science Topic Team
SVM : SerVice Module
SW : Solar Wind
SWCX : Solar Wind Charge eXchange
SWG : Science Working Group
SWP : Synergy White Paper
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
SciRD : Science Requirements Document
SiC : Silicon-Carbide
SoW : Statement of Work
T/O : Trade-Off
TAA : Technical Assistance Agreement
TAC : Time Allocation Committee
TAI : International Atomic Time
TAM : Transient Analysis Module (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
TBW : To Be Written
TCB : Barycentric Coordinate Time
TCM : Trajectory Correction Manoeuvre
TCS : Thermal-Control Sub-system
TCWG : Telescope Calibration Working Group
TDA : Technology Demonstrator Activity
TDB : Barycentric Dynamical Time
TDM : Time-Domain Multiplexing
TDP : Technology Development Plan
TEC-EES : space Environments and Effects Section in ESA's TEChnical Directorate
TECNET : TEChnology NETwork (ESA)
TES : Transition Edge Sensor
TF : Thermal Filter
TF : Transfer Function
THESEUS : Transient High-Energy Sky and Early-Universe Surveyor 
TID : Total Ionising Dose
TIM : Technical Interchange Meeting
TP : Technology Plan
TP : Topical Panel
TPA : Telescope Performance Assessment
TRA : Technology Readiness Assessment
TRD : Test Requirements Document
TRL : Technology Readiness Level
TRoPIC : single photon counting CCD camera, eROSITA prototype camera
TT : Terrestrial Time
TTC : Telemetry and TeleCommand
TTV : Total Thickness Variation
TV : Thermal Vacuum
TVAC : Thermal-VACuum (test / chamber)
TWG : Telescope Working Group
TWTA : Travelling WaveTube Amplifier
Te2v : Teledyne e2v
ToF : Time of Flight
ToO : Target of Opportunity
UFO : Ultra-Fast Outflow
UKSA : United Kingdom Space Agency
ULIRG : Ultra-Luminous InfraRed Galaxy
ULX : Ultra-Luminous X-ray (source)
URD : User Requirements Document
US : United States
USD : United States Dollar
UTC : Universal Coordinated Time
VERITAS : VErsatile Readout based on Integrated Trapezoidal Analogue Shapers
VF : Vignetting Function (Vignetting Factor)
VG : ViewGraph
VO : Virtual Observatory
VTT : Valtion Teknillinen Tutkimuskeskus (Finland's State Technical Research Centre)
W : Window (mode)
W1 : Wolter-1 (design)
WA : Wolter(-1) Athena
WBS : Work Breakdown Structure
WDA : WFI Data Analysis (software)
WEDM : Wire(-cut) Electrical Discharge Machining
WFEE : Warm Front-End Electronics
WFI : Wide-Field Imager
WFOIS : Wide-Field Optical/Infrared Surveyors
WG : Working Group
WHIM : Warm-Hot Inter-galactic Medium
WISC : WFI Science and data Centre
WOLTER : Mid-radius SPO TDA
WP : Work Package
WPD : Work Package Description
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
XCaT : X-IFU Calibration Team
XCoB : X-IFU Consortium Board
XDA : X-IFU Data Analysis (software)
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 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
XSAT : X-IFU Science Advisory Team
XSS : X-ray Source System
aLIGO : Advanced LIGO
aVirgo : Advanced Virgo
dMCR : delta Mission Consolidation Review (2017)
e2ePA : end-to-end Performance Assessment
f2f : face-to-face (meeting)
kSZ : kinetic SZ effect
ks : kilo-second
rtHz : square root of Hertz
tSZ : thermal SZ effect
uTCA : micro Telecommunications Computing Architecture (connector)
um : micrometer
z : redshift