| U-SB | unified S-band |
| U/B | usage block |
| U/C | undercurrent |
| U/C | user code |
| U/D | update |
| U/F | unique file |
| U/IP | user/international partner |
| U/L | uplink |
| U/M | unit of measure |
| U/M | unmanned |
| U/M | unscheduled maintenance |
| U/O | used on |
| U/V | undervoltage |
| U/W | used with |
| UA | unexplained anomaly |
| UAC | unattended assembly call |
| UAD | user accommodation document |
| UAL | United Airlines |
| UARS | upper atmosphere research satellite |
| UART | universal asynchronous receiver transmitter |
| UAW | United Auto Workers |
| UB | upper brace |
| UB | utility bridge |
| UBER | undetected bit error rate |
| UC | unsatisfactory condition |
| UCAR | unsatisfactory condition action report |
| UCC | universal checkout console |
| UCD | urine collection device |
| UCN | uniform control number |
| UCR | unsatisfactory condition report |
| UCS | universal control system |
| UCS | utilities control system |
| UCTA | urine collection transfer assembly |
| UDB | update buffer |
| UDC | utility distribution center |
| UDF | unit development folder |
| UDF | utility and data flow |
| UDL | up-data link |
| UDL | update link |
| UDMH | unsymmetrical dimethylhydrazine |
| UDMH/H | unsymmetrical dimethylhydrazine hydrazine blend |
| UDOP | ultrahigh Doppler |
| UDS | universal documentation system |
| UDS | utility distribution system |
| UE | user equipment |
| UER | unique-equipment register |
| UFD | user file directory |
| UFF | upper forward fuselage |
| UHF | ultrahigh frequency |
| UHT | umbilical handling technician |
| UI | unit of issue |
| UI | user interface |
| UIIP | user integration implementation plan |
| UIMS | user interface management system |
| UIWG | User Interface Working Group |
| UK | United Kingdom |
| UL | underload |
| UL | uplink |
| UL | upper left (S-band antenna) |
| UL | upper limit |
| ULA | Fairbanks, Alaska (STDN site) |
| ULC | unpressurized logistics carrier |
| ULCI | unpressurized logistics carrier interface |
| ULE | ultralow expansion (quartz) |
| ULI | Underwriters' Laboratories, Incorporated |
| ULM | unpressurized logistics module |
| ULO | unmanned launch operations |
| ULS | upload station |
| ULV | unmanned launch vehicle |
| UM | unmanned |
| UM | unscheduled maintenance |
| UMIF | upper mechanical integration frame |
| UMO | unmanned orbital |
| UMPS | up mission processing start |
| UMVF | unmanned vertical flight |
| UNEP | United Nations Environmental Program |
| UPF | universal patch format |
| UPGWA | United Plant Guard Workers of America |
| UPL | unapproved parts list |
| UPN | unique program number |
| UPP | user parameter processor (processing) |
| UPS | uninterruptible power system |
| UPS | upright perigee stage |
| UPWT | unitary plan wind tunnel |
| UR | unsatisfactory report |
| UR&C | usual, reasonable, and customary |
| URA | urine receptacle assembly |
| URE | user range error |
| US | upper stage |
| US | user segment |
| USA-2D/3D | unified solutions algorithm 2-dimensional/3-dimensional |
| USAF | United States Air Force |
| USASDC | U.S. Army Strategic Defense Command |
| USASDG | U.S. Army Space Defense Group |
| USB | unified S-band |
| USB | upper side band |
| USBE | unified S-band equipment |
| USBI | United Space Booster, Incorporated |
| USBS | unified S-band system |
| USC | upper stage contractor |
| USCG | United States Coast Guard |
| USE | unique support equipment |
| USE | unobtrusive sensors and effectors |
| USE | user support environment |
| USEIT | user station evaluation integration and test |
| USFWS | United States Fish and Wildlife Service |
| USI | update software identity |
| USI | user system interface |
| USML-1 | United States Microgravity Laboratory-1 |
| USNS | United States Navy ship |
| USO | ultrastable oscillator |
| USP | uniform specification program |
| USPHS | United States Public Health Service |
| USRA | Universities Space Research Association |
| USS | United States ship |
| USS | United States standard |
| USS | umbilical separation system |
| USS | unmanned space satellite |
| USS | upper stage system (PAM-D and interfaces) |
| USS | utility support structure |
| USSPACECOM | United States Space Command |
| USSR | Union of Soviet Socialist Republics |
| USVMS | urine sample volume measuring system |
| USWA | United Steelworkers of America |
| USWB | United States Weather Bureau |
| UT | unit tester |
| UT | universal time |
| UTC | United Technologies Corporation |
| UTC | United Technology Center |
| UTC | universal test console |
| UTC | universal time compensated |
| UTC | user time clock |
| UTE | universal test equipment |
| UTF | Underwater Test Facility |
| UTLM | up telemetry |
| UTRS | universal trouble reporting system |
| UTS | universal test set |
| UTS | unmanned teleoperator spacecraft |
| UTS | urine transfer system |
| UUT | unit under test |
| UV | ultraviolet |
| UV | undervoltage |
| UVAT | ultraviolet automatic tracker |
| UVD | undervoltage device |
| UVF | unmanned vertical flight |
| UVMS | urine volume measuring system |
| UVPE | UV/video processing electronics |
| UVS | ultraviolet sensor |
| UVW | quasi-inertial, right-handed Cartesian coordinate system |
| Unis | Unified Information System |