The letter V (or V band) is commonly used to indicate a passband located within the visible light range (V for visual). 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, sometimes similar.
I've seen "V" explained as "violet", but in the UBV system, it is outside the violet range.