Exposure Value can simply be thought of as an amount of light. EV 0 is less light, EV19 is more light. Each increase by one in EV is a doubling of the amount of light. EV is often expressed as a combination of shutter speed, aperture, and film speed. I believe the EV standard is for 100 speed film, but I'm uncertain of what changing film speed does to EV -- after all, the amount of light hasn't changed! With film speed ISO 100, EV0 corresponds to a shutter speed of 1 second and an aperture of f/1.0. You can vary shutter speed and aperture inversely with each other, and the EV will remain constant. I could say that EV = aperture * shutter_speed. Any combination of aperture and shutter speeds that results in the same EV means that their is that much light.

The Exposure Value (EV) system, which originated in Germany in the 1950s, was created to be a simple-to-use substitute for the shutter speed/aperture combination, using a single number instead of two.

An arbitrary method of incremental measurement that can be adjusted on some digital cameras for more precise exposure control.

Scale of values used to indicate the sensitivity range of a TTL or off-camera meter system within which accurate exposure measurement is guaranteed.

In photography, exposure value (EV) denotes all combinations of camera shutter speed and relative aperture that give the same exposure. The concept was developed in Germany in the 1950s (Ray 2000), in attempt to simplify choosing among combinations of equivalent camera settings. Exposure value also is used to indicate an interval on the photographic exposure scale, with 1 EV corresponding to a standard power-of-2 exposure step, commonly referred to as a “stop.” In optics, the term “stop” properly refers to the aperture itself, while the term “step” refers to a division of the exposure scale.