Exposure index (E.I.)
From The Glossary of Digital Photography
A numerical rating of the sensitivity of a film or a digital sensor to light. The higher the number, the more sensitive the film or sensor. E.I. is still sometimes called film speed, even when used to represent the sensitivity of the digital sensor. It is a numerical scale in which a doubling of a value represents a doubling of the sensitivity. Most digital cameras have a range of 100 to 800 or to 1,600. Exposure index is the same as ISO. In the past, ASA was the term used. See also ASA, ISO
