In gridiron football, a sack occurs when the quarterback is tackled behind the line of scrimmage before throwing a forward pass, when the quarterback is tackled behind the line of scrimmage in the "pocket" and without clear intent, or when a passer runs out of bounds behind the line of scrimmage due to defensive pressure. This often occurs if the opposing team's defensive line, linebackers or defensive backs are able to apply pass pressure to quickly get past blocking players of the offensive team, or if the quarterback is unable to find a back to hand the ball off to or an available eligible receiver to catch the ball, allowing the defense a longer opportunity to tackle the quarterback.

The quarterback, colloquially known as the "signal caller", is a position in gridiron football. Quarterbacks are members of the offensive platoon and mostly line up directly behind the offensive line. In modern American football, the quarterback is usually considered the leader of the offense, and is often responsible for calling the play in the huddle. The quarterback also touches the ball on almost every offensive play, and is almost always the offensive player that throws forward passes. When the QB is tackled behind the line of scrimmage, it is called a sack.