| U | Uranium | - | Astronomy |
| U?K | Initialism For Two Albanian Guerilla Movements | - | Astronomy |
| UA | Ukraine (ISO 3166 Digram)Unit Of Action | - | Astronomy |
| UAB | University Of Alabama At Birmingham | - | Astronomy |
| UAE | United Arab Emirates | - | Astronomy |
| UAH | Ukrainian Hryvnia (ISO 4217 Currency Code) | - | Astronomy |
| UAR | Uniformly At Random (probability) - United Arab Republic - Uni›n Argentina De Rugby (Spanish For Argentine Rugby Union) | - | Astronomy |
| UAV | Unmanned Aerial Vehicle - Unprotected Anal And Vaginal Sex | - | Astronomy |
| UBC | United Baptist Church - University Of British Columbia | - | Astronomy |
| UBIGEO | Ubicaci›n Geografica (Spanish For Geographical Location) | - | Astronomy |
| UBS | Unbundled Bitstream Services - United Bank Of Switzerland - United Banque Suisse | - | Astronomy |
| UCAV | Unmanned Combat Air Vehicle | - | Astronomy |
| UCBSA | United Cricket Board Of South Africa | - | Astronomy |
| UCF | University Of Central Florida | - | Astronomy |
| UCI | Union Cycliste Internationale (French | - | Astronomy |
| UCLA | University Of California | - | Astronomy |
| UCLES | University Of Cambridge Local Examinations Syndicate | - | Astronomy |
| UCR | University Of Costa Rica | - | Astronomy |
| UCS | Union Of Concerned Scientists | - | Astronomy |
| UDAS | Uppsala-DLR Asteroid Survey | - | Astronomy |
| UDDI | Universal Description | - | Astronomy |
| UDP | User Datagram Protocol | - | Astronomy |
| UDT | Underwater Demolition Team | - | Astronomy |
| UDTS | Uppsala-DLR Trojan Survey | - | Astronomy |
| UE | Unit Of Employment | - | Astronomy |
| UEFA | Union Of European Football Associations | - | Astronomy |
| UESAC | Uppsala-ESO Survey Of Asteroids And Comets | - | Astronomy |
| UFC | Ultimate Fighting Championship | - | Astronomy |
| UFO | Unidentified Flying Object - United Farmers Of Ontario | - | Astronomy |
| UG | Uganda (ISO 3166 And FIPS 10-4 Country Code Digram) | - | Astronomy |
| UG | Uyghur Language (ISO 639-1 Code) | - | Astronomy |
| UGA | Uganda (ISO 3166 Trigram) | - | Astronomy |
| UGV | Unmanned Ground Vehicle | - | Astronomy |
| UGX | Ugandan Shilling (ISO 4217 Currency Code) | - | Astronomy |
| UHF | Ultra High Frequency | - | Astronomy |
| UHMW | Ultra-High Molecular Weight | - | Astronomy |
| UHRB | Ultra-High Resolution Building (simulation) | - | Astronomy |
| UIC | University Of Illinois At Chicago | - | Astronomy |
| UID | User ID | - | Astronomy |
| UIG | Uyghur Language (ISO 639-2 Code) | - | Astronomy |
| UIUC | University Of Illinois At Urbana-Champaign | - | Astronomy |
| UJTL | Universal Joint Task List | - | Astronomy |
| UK | United Kingdom (alsoFIPS 10-4 Country Code) - University Of Kentucky | - | Astronomy |
| UK | Ukrainian Language (ISO 639-1 Code) | - | Astronomy |
| UKR | Ukraine (ISO 3166 Trigram) | - | Astronomy |
| UKR | Ukrainian Language (ISO 639-2 Code) | - | Astronomy |
| UL | Underwriters Laboratories | - | Astronomy |
| ULEB | Union Des Ligues Europ?ens De Basket-ball | - | Astronomy |
| ULF | Ultra Low Frequency | - | Astronomy |
| UM | U.S. Minor Outlying Islands (postal Symbol; ISO 3166 Digram) | - | Astronomy |
| UMBC | University Of Maryland Baltimore County | - | Astronomy |
| UMC | United Microelectronics Corporation | - | Astronomy |
| UMCP | Unit Maintenance Collection Point | - | Astronomy |
| UMES | University Of Maryland Eastern Shore | - | Astronomy |
| UMI | U.S. Minor Outlying Islands (ISO 3166 Trigram) | - | Astronomy |
| UMIST | University Of Manchester Institute Of Science And Technology | - | Astronomy |
| UMK | Uniwersytet Miko?aja Kopernika (Polish Nicolaus Copernicus University) | - | Astronomy |
| UML | Unified Marxist-Leninist Communist Party Of Nepal - Unified Modeling Language - University Of Massachusetts Lowell - User-Mode Linux | - | Astronomy |
| UMTS | Universal Mobile Telecommunications System | - | Astronomy |
| UN | United Nations | - | Astronomy |
| UNABOM | University And Airline Bomber (FBI Case Name) | - | Astronomy |
| UNAIDS | Joint United Nations Programme On HIV/AIDS | - | Astronomy |
| UNAMSIL | United Nations Mission In Sierra Leone | - | Astronomy |
| UNCF | United Negro College Fund | - | Astronomy |
| UNCLOS | United Nations Convention On The Law Of The Sea | - | Astronomy |
| UNCSGN | United Nations Conference On The Standardization Of Geographical Names | - | Astronomy |
| UNCTAD | United Nations Conference On Trade And Development | - | Astronomy |
| UNDCP | United Nations Drug Control Programme | - | Astronomy |
| UNDE | Union Of National Defence Employees (Canada) | - | Astronomy |
| UNDP | United Nations Development Programme | - | Astronomy |
| UNEP | United Nations Environment Programme | - | Astronomy |
| UNESCO | United Nations Educational | - | Astronomy |
| UNFCCC | United Nations Framework Convention On Climate Change | - | Astronomy |
| UNFICYP | United Nations Peacekeeping Force In Cyprus | - | Astronomy |
| UNGEGN | United Nations Group Of Experts On Geographic Names | - | Astronomy |
| UNHCR | United Nations High Commission(er) For Refugees | - | Astronomy |
| UNICEF | United Nations Children's Fund (originally The United Nations International Children's Emergency Fund) | - | Astronomy |
| UNICRI | United Nations Interregional Crime And Justice Research Institute | - | Astronomy |
| UNIDO | United Nations Industrial Development Organization | - | Astronomy |
| UNIFEM | United Nations Development Fund For Women | - | Astronomy |
| UNIS | University Of Surrey | - | Astronomy |
| UNLV | University Of Nevada | - | Astronomy |
| UNMIK | United Nations Interim Administration Mission In Kosovo | - | Astronomy |
| UNMOVIC | United Nations Monitoring | - | Astronomy |
| UNOSOM | United Nations Operations In Somalia | - | Astronomy |
| UNPROFOR | United Nations Protection Force (in Former Yugoslavia) | - | Astronomy |
| UNRISD | United Nations Research Institute For Social Development | - | Astronomy |
| UNSM | Union Of Nova Scotian Municipalities | - | Astronomy |
| UNTAC | United Nations Transitional Authority In Cambodia | - | Astronomy |
| UNTAET | United Nations Transitional Administration In East Timor | - | Astronomy |
| UNU | United Nations University | - | Astronomy |
| UO | Urban Operations | - | Astronomy |
| UOG | University Of Guelph | - | Astronomy |
| UP | Ukraine (FIPS 10-4 Country Code) -Uttar Pradesh | - | Astronomy |
| UPC | Universal Product Code | - | Astronomy |
| UPDM | Uganda People?s Democratic Movement - Unified Modeling Language Profile For DoDAF And MoDAF - Unit? De Psychiatrie Du D?veloppement Mental - Universal Pattern Decomposition Method - Unix Power Device Management - Unpressurized Docking Mast - Urban Planni | - | Astronomy |
| UPDM | Universit? Popolare Della Multimedialit? | - | Astronomy |
| UPF | Universitat Pompeu Fabra (Catalan | - | Astronomy |
| UPI | United Press International | - | Astronomy |
| UPN | United Paramount Network | - | Astronomy |
