(photometric filter for ultraviolet light)
U stands for the ultraviolet passband,
or a filter implementing that passband.
It is one of the passbands/filters of the UBV photometric system,
in which it is centered around wavelength 365 nm,
with a width of 66 nm (full width at half maximum).
The letter is also used to indicate the magnitude of the signal
observed through the filter, e.g., citing "u = 1.5".
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SkyMapper Southern Survey (SMSS)
UBV photometric system