| Acronym | Meaning |
 | ULEV | Ultra Low Emission Vehicle |
| ULF | Ultra Low Frequency |
| ULFM | Unified Ligand Field Model (for Mol. Electronic Energies) |
| ULIRG | Ultraluminous Infrared Galaxy |
| ULPA | Ultra Low Penetration Air |
| ULSI | Ultra-large-scale Integration |
| ULTRA | Underground Leak Transport Assessment Model |
| UMDE | Ultramicrodisk Electrode |
| UMEA | Ultramicroelectrode Array |
| UMER | Ultrasonically Modulated Electron Resonance |
| UMINDO | Spin-unrestricted Modified Intermediate Neglect Of Differential Overlap |
| UMIX | User-Manufacturer Information EXchange |
| UMLS | Unified Medical Language System |
| UMNDO | Spin-unrestricted Modified Neglect Of Diatomic Overlap |
| UMO | Unmodified Molecular Orbital |
| UMP | Unrestricted Moeller-Plesset |
| UMP | Uridine 5'-phosphate |
| UMP2 | Unrestricted Moeller-Plesset 2nd-order (perturbation Theory) |
| UMP4SDTQ | Unrestricted Moeller-Plesset 4th-order (perturbation Theory With) Single |
| UMTRCA | Uranium Mill Tailings Radiation Control Act |
| UNAMAP | Users' Network For Applied Modeling Of Air Pollution |
| UNCED | United Nations Conference On Environment And Development |
| UNDS | Uniform National Discharge Standards |
| UNECE | United Nations Economic Commission For Europe |
| UNEP | United Nations Environmental Program |
| UNESCO | United Nations Educational |
| UNFCCC | United Nations Framework Convention On Climate Change |
| UNIDO | United Nations Industrial Development Organization |
| UNIFAC | Universal Quasichem. Functional Group Activity Coeffs. |
| UNIRAM | Software For Reliability |
| UNITAR | United Nations Institute For Training And Research |
| UNO | Unrestricted Natural Orbital |
| UNO-CAS | Unrestricted Natural Orbital-complete Active Space |
| UNOS | United Network For Organ Sharing |
| UNS | Unified Numbering System For Metals And Alloys |
| UNSCEAR | United Nations Scientific Committee On The Effects Of Atomic Radiation |
| UOFMN | Unified Organically Functionalized Mesoporous Network |
| UOP | Universal Oil Products |
| UOT | Universal Oxidn. Test |
| UOVMO | Unoccupied Valence-type Mol. Orbital |
| UPA | Unitary-pole Approximation |
| UPD | Underpotential Deposition |
| UPES | Ultraviolet Photoelectron Spectroscopy |
| UPF | UV Protection Factor |
| UPOT | Union Pacific Oxidation Test |
| UPS | UV Photoelectron Spectroscopy |
| UPS | Uninterrupted Power Systems (for Computer Networks) |
| UPTF | Upper Plenum Test Facility |
| UPVC | Unplasticized Poly(vinyl Chloride) |
| UPW | Ultrapure Water |
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