| Abbreviation | Definition |
|---|---|
| UEF | Unidentified Energy Factor |
| UESAC | UPPSALA ESO Survey of Asteroids and Comets |
| UFO | Unidentified Flying Object |
| UGC | Upsala General Catalog |
| UGSS | U Gem variable type star (SS Cygnus subtype) |
| UGWZ | U Gem variable type star (WZ Sagittae sub-type) |
| UGZ | U Gem variable type star (Z Camelopardalis subtype) |
| UHTC | Ultra High Transmission Coatings |
| UKSA | United Kingdom Space Agency |
| ULD | Ultra-wide Low- Distortion lens |
| ULYSSES | Ultraviolet LibrarY of Spectra for Star-forming Environments |
| UMP | Ultra Metal-Poor |
| UMU | Extravehicular Mobility Unit |
| UNICELLAR | Utilitarian Nuclear Isotopian Crude Extrusion Liquid Lateral Adobe Reality |
| URO | Unidentified Red Object |
| URP | Uranian Ring Plane |
| URPX | Uranian Ring Plane Crossing |
| USS | Unusual Star Shape |
| UVOT | Ultraviolet Optical Telescope |
| UV-VIS | Ultraviolet - Visible |