I've read lots of definitions and I think I understand what EV might be, but I wish the definitions where simpler to explain, and even better that they might at least be more consistent!
EV appears to be a common way that digital camera manufacturers have chosen to "measure" and control the sensitivity of the image sensor, the aperture and shutter speed. It is the digital equivalent of the ISO, or ASA, film speed. At the most technical level it is a single number that quantifies the amount of light for a particular exposure. It can be used by the cameras light meter and electronics programming to calculate the correct combination of shutter speed and aperture. In the simplest terms it is a attempt to measure the brightness of a scene. Having agonized about explaining all that, the absolute value is not something you will normally see displayed or recorded. What you may find is EV used as an exposure adjustment. Unlike film, the sensitivity of digital cameras, can be adjusted.
Many Cameras now come with simple up and down adjustments that let you change the EV setting. Positive adjustment will lighten an image. Negative adjustments will darken the image. Some camera might use an ISO equivalent adjustment as well or in place on an EV adjustment. The key thing to remember is you don't have to change film to change the sensitivity of you camera to changes in lighting