| U dk | Upper deck |
| U/S | Unserviceable |
| UAC | Upper air control centre |
| UAR | Upper air route |
| UART | Universal asynchronous receiver/transmitter |
| UAS | Upper air space |
| UAV | Unmanned aerial vehicle |
| UBE | Ultra bypass engine (aircraft) |
| UCPS | Unhardened collective protection system |
| UDDI | Universal description, discovery and integration (computing) |
| UDF | Unducted fan (aircraft engine) |
| UDT | Underwater destruction team |
| UDU | Underwater demolition unit |
| Underwater destruction unit |
| UE | Unintentional emission (radar) |
| UFC | Up-front control |
| UFN | Until further notice |
| UGS | Unattended ground sensor |
| UHB | Ultra-high bypass (gas turbine) |
| UHCA | Ultra-high-capacity aircraft |
| UHF | Ultra-high frequency (300 to 3 000 MHz) |
| UHV | Ultra-high vacuum |
| UIC | Unit identification code |
| UIR | Upper flight information region |
| UK | United Kingdom |
| UKAATS | UK advanced air traffic system |
| UKADGE | UK air defence ground environment |
| UKADR | UK NATO air defence region |
| UKADS | UK air defence system |
| UKAEA | United Kingdom Atomic Energy Authority |
| UKAS | United Kingdom Accreditation Service |
| UKCS | United Kingdom continental shelf |
| UKKM | United Kingdom key management (communications) |
| UKMF | United Kingdom Mobile Force |
| ULA | Uncommitted logic array |
| ULCC | Ultra-large crude carrier |
| ULD | Unit load device |
| ULW | Unladen weight |
| UMA | Unmanned aircraft |
| UMBA | Universal military breathing apparatus |
| UMF | Unconventional military forces |
| UMS | Unattended machinery space |
| UMTS | Universal mobile telephone system |
| UN | United Nations |
| Un dk | Under deck |
| UNCITRAL | United Nations Commission on International Trade Law |
| unclass | Unclassified |
| UNCLOS | United Nations Conference on the Law of the Sea |
| UNCTAD | United Nations Conference on Trade and Development |
| UNDP | United Nations Development Programme |
| UNEP | United Nations Environment Programme |
| UNESCO | United Nations Educational, Scientific and Cultural Organisation |
| UNIDO | United Nations Industrial Development Organisation |
| unl | Unlimited |
| UNO | United Nations Organisation |
| UO | Underwriter´s office |
| UOR | Urgent operational requirement |
| UPC | Universal product code |
| upr | Upper |
| UPS | Uninterruptible power supply |
| Universal polar stereographic (grid, nav) |
| UPU | Universal Postal Union |
| URD | User requirements document |
| URG | Underway replenishment group |
| URL | Universal resource locator (internet) |
| USA | United States of America |
| USART | Universal synchronous/asynchronous receiver/transmitter |
| USB | Universal serial bus (computer) |
| Upper side band (radio) |
| USGW | Underwater-to-surface guided weapon |
| USV | Unmanned surface vehicle |
| UT | Universal time |
| UTA | Upper terminal area |
| UTC | Universal time, co-ordinated |
| UTM | Universal transverse mercator (projection grid) |
| UTP | Unshielded twisted pair (wiring) |
| UUT | Unit under test |
| UUV | Unmanned underwater vehicle |
| UV | Ultraviolet |
| UWB | Ultra-wideband (radio) |
| UWT | Underwater telephone |
| UXB | Unexploded bomb |
| UXO | Unexploded ordnance |
