Exposure value (EV)
From The Glossary of Digital Photography
A number assigned to a particular exposure level no matter what combination of shutter speed and aperture are used to get to that exposure. By convention, EV 0 is set at an aperture of f /1.0 and a shutter speed of 1 second. Each increase of one in the EV corresponds to a halving of the amount of light. So each step of EV corresponds with a step in aperture or shutter speed. Light meters and some cameras have markings for EV.
