| Term | Definition / Meaning / Stand for |
| USC SERC |
Space Engineering Research Center at USC |
| URAN |
Ukrainian Radio Interferometer Network |
| URCA |
Ultra Rapid Cooling Approximation |
| UDS |
Ultra-Deep Survey |
| UDG |
Ultra-Diffuse Galaxy |
| UFS |
Ultra-Fast Spectroscopy |
| ULF-WAVES |
Ultra-Low Frequency Waves in Space Plasma |
| ULV |
Ultra-Low Velocity |
| ULISS |
Ultra-Luminous Infrared Spectroscopic Survey |
| UST |
Ultra-Short-Term Variability |
| UTR |
Ultra-Transmittance Reflection |
| UVB |
Ultraviolet B Radiation |
| UEML |
Ultraviolet Emission Line Mapping |
| UVIT |
Ultraviolet Imaging Telescope |
| UVI |
Ultraviolet Imaging |
| UVM |
Ultraviolet Mapping |
| URE |
Ultraviolet Radiation Exposure |
| UR |
Ultraviolet Radiation |
| UVR |
Ultraviolet Radiation |
| USP |
Ultraviolet Spectroscopy |
| UFOC |
Unidentified Flying Object Catalog |
| UN COPUOS |
United Nations Committee on the Peaceful Uses of Outer Space |
| USI |
United States International Space Program |
| USST |
United States Space Telescope |
| UPS |
Universite Paris-Saclay (space science) |
| UNI |
University Network for Innovation in Space |
| UO |
University Observatory |
| UOA |
University of Arizona (astronomy research) |
| UAT |
University of Arizona Telescope |
| UF |
University of Florida (space research) |
| UNH EOS |
University of New Hampshire Earth Observation System |
| UOAC |
University of Oxford Astrophysics Center |
| UPRM |
University of Puerto Rico Mayaguez (space research) |
| USC ASTRO |
University of Southern California Astronomy Department |
| USC NASA |
University of Southern California NASA Lab |
| UTAS |
University of Tasmania (space research) |
| UTokyo |
University of Tokyo (space research) |
| UWAO |
University of Western Australia Observatory |
| UFS |
University of the Free State (space studies) |
| UGC |
Uppsala General Catalog of Galaxies |
| UMA |
Ursa Major (constellation) |
| UMI |
Ursa Minor (constellation) |
| UNIST |
Ulsan National Institute of Science and Technology (space science) |
| UCD |
Ultra-Compact Dwarf |
| ULF |
Ultra-Low Frequency |
| ULIRG |
Ultra-Luminous Infrared Galaxy |
| ULX |
Ultra-Luminous X-ray Source |
| UVD |
Ultraviolet Detector |
| UVOT |
Ultraviolet Optical Telescope |
| UVS |
Ultraviolet Spectrometer |
| UV |
Ultraviolet |
| UXO |
Unexploded Ordnance (space debris) |
| UFO |
Unidentified Flying Object |
| UKIRT |
United Kingdom Infrared Telescope |
| UKSA |
United Kingdom Space Agency |
| UNOOSA |
United Nations Office for Outer Space Affairs |
| UN |
United Nations (space treaties and policies) |
| USAF |
United States Air Force (space programs) |
| USGS |
United States Geological Survey (planetary mapping) |
| USNO |
United States Naval Observatory |
| USN |
United States Navy (space research) |
| USA |
United States of America (space exploration leader) |
| UCM |
Universidad Complutense de Madrid (space science) |
| UNSA |
Universidad Nacional de San Agustin (space research) |
| UPF |
Universitat Pompeu Fabra (astronomy research) |
| USRA |
Universities Space Research Association |
| UCL |
University College London (astronomy research) |
| UOB |
University of Birmingham (space research) |
| UCB |
University of California, Berkeley (astronomy research) |
| UCLA |
University of California, Los Angeles (space research) |