| Acronym | Meaning |
| TWSM | Two-way Shape Memory |
| TWTA | Traveling Wave Tube Amplifier |
| TWTDS | Treatment Works Treating Domestic Sewage |
| TWV | Trinkwasserverordnung (German Drinking Water Ordinance) |
| TXRF | Total Reflection X-ray Fluorescence |
| TZ | Triple Zeta (basis Set) |
| TZP | Tetragonal Zirconia Polycrystal |
| U.S.C. | United States Code |
| UA | Unequilibrated Aggregate (interplanetary Dust Particle) |
| UA | United Atom |
| UAM | Urban Airshed Model |
| UAM-IV | Urban Airshed Model Version IV |
| UAMO | Unprojected Alternant Molecular Orbital |
| UAP | Utilization Associated Products |
| UAPS | Utility Acid Precipitation Study |
| UAPSP | Utility Acid Precipitation Study Program |
| UAQI | Uniform Air Quality Index |
| UARP | Upper Atmospheric Research Program |
| UARS | Upper Atmosphere Research Satellite |
| UASB | Upflow Anaerobic Sludge Blanket |
| UASBR | Upflow Anaerobic Sludge Blanket Reactor |
| UATMP | Urban Air Toxics Monitoring Program |
| UBA | Unitarized Born Approxn. |
| UBD | Under-bath Decarburization |
| UBFF | Urey-Bradley Force Field |
| UBM | Under Bump Metallurgy |
| UBT | Urea Breath Test |
| UCA | Unit Cell Approximation [simulation Method] |
| UCAR | University Corporation For Atmospheric Research |
| UCAS-SCF | Unrestricted Complete Active Space Self-consistent Field |
| UCC | Unitary Coupled-cluster |
| UCC(N) | Unitary Coupled-cluster [truncated To Nth Order] |
| UCCD | (spin) Unrestricted Coupled Cluster (with) Double (excitations) |
| UCCSD | (spin) Unrestricted CCSD |
| UCDW | Ultrarelativistic Continuum Distorted Wave |
| UCEPA | Unitary Coupled Electron Pair Approximation |
| UCG | Underground Coal Gasification |
| UCHF | Uncoupled Hartree-Fock |
| UCHFPT | Uncoupled Hartree-Fock Perturbation Theory |
| UCI | Ultrasonic Contact Impedance |
| UCID | Unrestricted CI (with) Single (and) Double (excitations) |
| UCL | Upper Confidence Limit |
| UCM | Unresolved Complex Mixt. |
| UCMP | Unrestricted Compact Model Potential |
| UCMR | Unregulated Contaminant Monitoring Regulation |
| UCP | Uncoupling Protein |
| UCS | Unconfined Compressive Strength |
| UCST | Upper Critical Solution Temperature |
| UCT | University Of Cape Town |
| UDDS | Urban Dynamometer Driving Schedule |