The magnitude of a star in ultraviolet light on the Johnson photometry system. The U filter has a central wavelength of 366 nm and a bandwidth of 65 nm. It is that part of the older photographic magnitude which contains the ultraviolet light. There are also U filters in Geneva, RGU, Vilnius, and Walraven photometry, and in the six-colour system, but the context should always make it clear when these are intended.
Subjects: Astronomy and Astrophysics.