American football
A safety is scored if a player causes the ball to become dead in his own end zone. When this happens two points are awarded to the opposing (usually defending) team.  This can happen if a player is either downed or goes out of bounds in the end zone while carrying the ball. It can also happen if he fumbles the ball, and it goes out of bounds in the end zone. A safety is also awarded to the defensive team if the offensive team commits a foul which is enforced in its own end zone.