The letter U (or U band) is commonly used to indicate a passband
located in the ultraviolet range (U for ultraviolet), typically
for one located not far from the visible light range. The
letter is also often used for a filter for the band (or more
explicitly, a U filter), and to indicate a magnitude of a signal
observed through such a filter, e.g., citing "U = 1.5".
In the conventional UBV photometric system and its offshoots,
the U band is centered around wavelength 365 nm, with a width
of 66 nm (full width at half maximum). Other systems may have different specifications,
in some cases, similar.