Sack
What is Sack?
A statistic in football credited to a defensive player or players when physical contact with a quarterback 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.
Sporting Charts explains Sack
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.
When one or more players are involved in a sack, each player is credited with half a sack. A successful sack results in yards being subtracted from a team's passing totals, but does not affect the total passing yards of the quarterback.
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.