| Acronym | Meaning |
| UDF | Unrestricted Dirac-Fock |
| UDI | Utility Data Institute (power Generation) |
| UDMH | Unsymmetrical Dimethyl Hydrazine (Alar Metabolite) |
| UDP | Uridine 5'-diphosphate |
| UDRI | University Of Dayton Research Institute |
| UDWA | Unitarized Distorted-wave Approximation |
| UDWBA | Unitarized Disorted Wave Born Approxn. (for Scattering) |
| UEATC | European Union For Technical Agreements In Construction (French; L'Union Europeenne Pour L'Agrement Technique Dans La Construc |
| UEBS | Unitarized Eikonal Born Series (in Electron-atom Scattering) |
| UEGO | Universal Exhaust Gas Oxygen Sensor |
| UEL | Upper Explosive Limit |
| UERPIC | Underground Excavation And Rock Properties Information Center |
| UF | Ultrafiltration (water Treatment) |
| UF | Uncertainty Factor (in Oral RfD) |
| UFC | Uniform Fire Code |
| UFF | Universal Force Field |
| UFFI | Urea-formaldehyde Foam Insulation |
| UFG | Ultrafine Grained |
| UFL | Upper Flammable Limit |
| UFM | Ultrasonic Force Microscopy |
| UFP | Ultrafine Particles |
| UGF | Unitary-group Formulation (in Many-body Quantum Theory) |
| UGLCC | Upper Great Lakes Connecting Channels |
| UHC | Unburned Hydrocarbons |
| UHE | Ultra-high Energy |
| UHF | Ultra-high Frequency |
| UHF | Unrestricted Hartree-Fock |
| UHF-AA | Unrestricted Hartree-Fock (with Spin) Annihilation |
| UHF-CO | Unrestricted Hartree-Fock Crystal Orbital |
| UHFQ | Unrestricted Hartree-Fock With Quartet Annihilation |
| UHFS | Unrestricted Hartree-Fock-slater |
| UHIPI | Ultrahigh Intensity Post-ionization |
| UHMPE | Ultra High Modulus Polyethylene |
| UHMWPE | Ultra High Molecular Weight Polyethylene |
| UHP | Ultra High Purity |
| UHPLC | Ultrahigh-pressure Liquid Chromatography |
| UHT | Ultra High Temperature |
| UHTS | Ultra High-throughput Screening |
| UHV | Ultra High Vacuum |
| UHVEM | Ultra High Voltage Electron Microscopy |
| UIC | Underground Injection Control |
| UIHRS | Upflow Inclined High-rate Settlers |
| UJT | Unijunction Transistor |
| UKAEA | United Kingdom Atomic Energy Authority |
| UKAS | United Kingdom Accreditation Service |
| UKAWRG | United Kingdom Acid Waters Review Group |
| UKB | Unrestricted Kinetic Balance (in Calcns. Of Wave Functions And Energies) |
| UKIRT | United Kingdom Infrared Telescope |
| UKMO | United Kingdom Meteorological Office |
| ULDPE | Ultra Low Density Polyethylene |