V stands for the Visual 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 551 nm, with a width of 88 nm (Full Width At Half Maximum). 551 nm is between green and yellow within the Visible Light spectrum, the width reaching from blue to orange.

The letter is also used to indicate the Magnitude of the signal observed through the filter, e.g., citing "v = 1.5".

I've seen "V" explained as "violet", but in the UBV system, it is outside the violet range.