A member of either team who lines up off of the line of scrimmage. On offense, backs consist of the quarterback and the running backs. On defense, the backs are made up of the cornerbacks and safeties. The number of backs on the field on either team changes depending on the formation that a team is utilizing during that particular play.
The position of back was used in the early years of football as a catch-all term for any player who was not a lineman. At the time, most offenses were primarily focused on running the football and would use a combination of backs to keep the defense off balance. The other reason for the term was that most players played on both offense and defense during the game, so they would be listed as a "back" in the game program since they would be playing in both the offensive and defensive backfields.