Also a
football (North American, not soccer) term, for when on a passing play, a
quarterback is tackled behind the
line of scrimmage by the
defense.
On obvious quarterback runs (such as quarterback draws or sneaks), if the QB is tackled behind the line of scrimmage, it is NOT a sack. Sacks are limited to losses in passing plays (or if the quarterback attempts to scramble when unable to pass the ball). Sacks are also credited if the quarterback is chased out of bounds behind the line of scrimmage.
Individually, the defensive player who sacks the quarterback is credited with a sack. In the case of two players, each is credited with 1/2 a sack (sacks are not split beyond 1/2s).
The sack became an official NFL stat in 1982. The all-time career sack leader is Reggie White, with 198. The NFL single-season sack record is held by Mark Gastineau (22 for the New York Jets in 1984).
"Sack" can be used as a noun or a verb ("to sack the quarterback").