What is a sack for statistical purposes?

A sack is scored when an apparent passing play ends with the potential passer stopped, out of bounds, or fumbling at or behind the statistical line of scrimmage. A designed rushing loss is scored differently.

For example, a quarterback dropping back and being tackled 6 yards behind the line takes a sack for a 6-yard loss, not an incomplete pass. Scorers judge the play's passing intent, so not every tackle of a quarterback is a sack.