| U-ACHX | UKC (United Kennel Club) Agility CHampion eXcellent 
| Others |
| U-AGI | UKC (United Kennel Club) AGility I
| Others |
| U-AGII | UKC (United Kennel Club) AGility II
| Others |
| U-boat | Undersea-boat
| Others |
| U-BOOT | Universal Boot Loader
| Others |
| U-CD | UKC (United Kennel Club) - Companion Dog
| Others |
| U-CDX | UKC (United Kennel Club) - Companion Dog Excellent
| Others |
| U-KISS | Ubiquitous Korean International Superstar
| Others |
| U-SB | Unified S-Band
| Others |
| U-UD | UKC (United Kennel Club) - Utility Dog
| Others |
| U-W | Underwriter
| Others |
| u. | and (und, [German])
| Others |
| u. | and [und (German)]
| Others |
| u. | under (unter, [German])
| Others |
| u. a. | und andere
| Others |
| u. a. | unter anderem
| Others |
| u. a. | unter anderen
| Others |
| u.a. | u. a.
| Others |
| U.C. | Unidad Central
| Others |
| U.D. | Ut Dictum (As Directed)
| Others |
| U.R.M.S | Unicorns Rock My Socks!
| Others |
| U.S.AFRICOM | United States Africa Command
| Others |
| U.S.C. | United States Code
| Others |
| u.s.w. | and so forth (und so weiter) [Ger.]
| Others |
| U/A | Under Advisement
| Others |
| U/A | Under Agreement
| Others |
| U/A | Universal and Adult
| Others |
| U/A | UrinAnalysis
| Others |
| U/B | Under Bore
| Others |
| U/C | Under Current
| Others |
| U/C | Unit Cost
| Others |
| U/C | Upconverter
| Others |
| U/F | Un-Furnished
| Others |
| U/F | Unit of Fire
| Others |
| U/g Sprk | Underground sprinkler
| Others |
| U/L | Units per Liter
| Others |
| U/L | Uplink
| Others |
| U/L | Uplink (UPLK preferred)
| Others |
| U/L | UpLoading
| Others |
| U/lc | Upper/Lower Case typeset
| Others |
| U/M | Unit of Measure
| Others |
| U/M | Unmanned
| Others |
| U/M | Unscheduled Maintenance
| Others |
| U/O | Under Observation for ....
| Others |
| U/O | Used on
| Others |
| U/Pb | uranium/lead
| Others |
| U/S | UltraSonic
| Others |
| U/S | UltraSound
| Others |
| U/S | UnderSection
| Others |
| U/S | Upper Stage
| Others |