| ultraviolet radiation | Electromagnetic radiation of wavelengths shorter than the shortest visible wavelengths; radiation of wavelengths in the approximate range 10 to 400 nm. |
| umbra | The central, completely dark part of a shadow. |
| uncertainty principle | Heisenberg uncertainty principle. It is fundamentally impossible to make simultaneous measurements of a particle's position and velocity with infinite accuracy. |
| universe | The totality of all matter, radiation and space; everything accessible to our observations. |
| variable star | A star that varies in luminosity. |
| vernal equinox | The point on the celestial sphere where the Sun crosses the celestial equator passing from south to north; a time in the course of the year when the day and night are roughly equal. |
| very-long-baseline interferometry (VLBI) | A technique of radio astronomy whereby signals from telescopes thousands of kilometers apart combined to obtain very high resolution by letting waves from different sites interfere with each other. |
| visual binary star | A binary star in which the two components are telescopically resolved. |
| void | A region between clusters and superclusters of galaxies that appears relatively empty of galaxies. |