In American football and Canadian football, a sack occurs when the quarterback (or another offensive player acting as a passer) is tackled behind the line of scrimmage before he can throw a forward pass, when the quarterback is tackled behind the line of scrimmage in the "pocket" and the intent of what he was going to do is unclear, 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 quickly get past blocking players of the offensive team (the quarterback's protection), or if the quarterback is unable to find a back to hand the ball off to or an available eligible receiver (including wide receivers, running backs and tight ends) to catch the ball, allowing the defense a longer opportunity to tackle the quarterback.
In the NFL, it is possible to record a sack for zero yards. The QB must pass the statistical line of scrimmage to avoid the sack. A sack is also credited when a defender causes the quarterback to fumble and the defending team recovers the ball behind or at the line of scrimmage. If a sack is done when the quarterback is in his own end zone, it is a safety and the defending team is awarded two points.
In the NFL yards lost on the play are added as negative yardage to the team's passing totals; however, the quarterback's individual passing total stats remains unchanged. Previously, tackling a player behind the line of scrimmage resulted in a loss from that player's (passing) statistics, even if it were fairly obvious a rush play had been intended. NCAA continues to subtract sack yardage from individual rushing totals.
To be considered a sack the quarterback must intend to throw a forward pass. If the play is designed for the quarterback to rush (run) the ball, any loss is subtracted from the quarterback's rushing total (and the play is ruled a Tackle-for-loss as opposed to a sack). If the quarterback's intent is not obvious, statisticians use certain criteria, such as the offensive line blocking scheme, to decide. Unique situations where a loss reduces a quarterback's rushing total (not a sack) are "kneel downs" (used to run time off the game clock).
A player will receive credit for half of a sack when multiple players contribute to the sacking of a quarterback, even if more than two players contributed.
The term "Quarterback sack" was first coined by Hall of Fame defensive end Deacon Jones. Jones, who played in the NFL from 1961 to 1974, felt that a sack devastated the offense in the same way that a city was devastated when it was sacked.
However, the term "sack" was not widely used before ca. 1970; previously one would simply refer to a player's being tackled behind the line (of scrimmage), in so many words. The NFL only began to keep track of times a quarterback was sacked since 1963 and sacks by defensive players in 1982. Team records have been kept at least since the 1940s.
- NFL Single Season Sacks: 22.5 Michael Strahan, 2001*
- NFL Career Sacks: 200 Bruce Smith, 1985-2003
- NFL Game Sacks: 7 Derrick Thomas, Nov. 11th, 1990
- NFL Sacks Rookie Season: 14.5 Jevon Kearse, 1999
- NFL Most Consecutive Games committing a sack: 69 Tampa Bay, 1999-2003
- Official in NFL since 1982
*Deacon Jones had 26 sacks in 14 games in the 1967 season, and 24 sacks in 14 games in the 1968 season. However, as sacks were not officially recorded by the NFL at that point, Michael Strahan is attributed the single season record.
See also 
-  2011 Football Statisticians Manual, p. 17.
- Deacon Jones from The Pro Football Hall of Fame
- Deacon of Defense - Pro Football Hall of Fame
- NFL Records
- NFL Records
- NFL Records
- NFL Career Sacked Leaders - Pro-Football-Reference.com
- NFL Single-Season Sacked Leaders - Pro-Football-Reference.com
- NFL Single Game Sacked Leaders - Pro-Football-Reference.com
- Sack Story, an article describing the controversy over the sack record