Definition: B-V: B-V color is one of the two color indices in the UBV photometric system.
Astronomers measure the amount of light a star gives off in different colors. Hot stars give off more blue light than red; cool stars give off more red light than blue. Colored filters are used to measure different wavelengths of light from stars. The magnitude of the star is measured first through a standardized B-band ("blue") filter. Then the star's magnitude is measured through a V-band ("visible", peaking in green) filter. The value of V is subtracted from B to get the B-V color index.