The letter V (or V band) is commonly used to indicate a passband
located within the visible light range (V for visual or visible). The
letter is also often used for a filter for the band (or more
explicitly, a V filter), and to indicate a magnitude of a
signal observed through such a filter, e.g., citing "V = 1.5".
In the conventional UBV photometric system and its offshoots,
the V band is centered around wavelength 551 nm, with a width
of 88 nm (full width at half maximum). Other systems may have different specifications,
in some cases, similar.
I've seen "V" explained as "violet", but in the UBV system,
it is outside the violet range; its central wavelength is
green.