An offensive statistic in football that is charged to a quarterback when physical contact by a defensive player or players causes him to be ruled down by contact behind the line of scrimmage, prior to an attempt to throw a forward pass or if the play is determined to resemble a passing play.
In order for a sack to be recorded, it must be obvious that the quarterback either intends to throw a forward pass, or is still in the "pocket" without a clear objective for the play. If officials determine that the play was intended as a rush, a tackle of the quarterback does not result in a sack and is recorded as negative rushing yardage by the quarterback.
The most common form of sack is when a defensive tackle, defensive end, or linebacker tackles a quarterback behind the line of scrimmage. However, if a defensive player makes any sort of physical contact with a quarterback (and conditions qualify for a sack), who is then ruled down by contact (such as falling to a knee), this is also counted as a sack. Quarterbacks sometimes "sack themselves" when moving around in or out of the pocket to avoid pressure-they trip or fall down and are then touched by a defensive player behind the line of scrimmage-this is then recorded as a sack.
The amount of times a quarterback has been sacked is a cumulative statistic and is sometimes represented as an average over a number of games when rating a team's pass protection.
A quarterback is also considered sacked if he loses a fumble on or behind the line of scrimmage that is recovered by the opposing team.