A term for any offensive formation that has only one running back. A one-back formation can have either a back lined up directly behind the quarterback or a back lined up next to the quarterback in a shotgun set. The back can be either a running back or a fullback, depending on whether the team is looking for a runner, receiver or blocker on a given play.
There is a large variety of one-back formations, although there are many similarities among them. One-back formations tend not to be used for power running plays due to the lack of a lead blocker. In some sets, a single back will line up behind the quarterback and be available for a running play or a passing play. In other sets, a back will line up next to the quarterback in a shotgun. While this tends to be more of a passing formation, the running back can still take a handoff on a draw.