Fumble (American football)

A fumble in American and Canadian football occurs when a player who has possession and control of the ball loses it before being downed (tackled) or scoring. By rule, it is any act other than passing, kicking, punting, or successful handing that results in loss of player possession. A fumble may be forced by a defensive player who either grabs or punches the ball or butts the ball with his helmet (a move called "tackling the ball"). A fumbled ball may be recovered and advanced by either team (except, in American football, after the two-minute warning in either half or 4th down, when the fumbling player is the only offensive player allowed to advance the ball, otherwise the ball is ruled dead at the spot of recovery if the ball bounces backwards or spotted at the point of the fumble if the ball travels forward). It is one of three events that can cause a turnover (the other two being an interception or on downs, though the latter does not count toward the team's total turnovers), where possession of the ball can change during play.

Under American rules a fumble may be confused with a muff. A muff occurs where a player drops a ball that he does not have possession of, such as while attempting to catch a lateral pass or improperly fielding a kicking play such as a punt (you cannot "fumble" a loose ball). Ball security is the ability of a player to maintain control over the football during play and thus avoid a fumble.

If the ball is fumbled the defensive team may recover the ball and even advance it to their opponents' goal. The same is true for the offense, but usually when the offense recovers the ball it simply tries to down it. In American football the offense cannot advance the ball if it recovers its own fumble on fourth down, or in the last two minutes of the game, unless the ball is recovered by the fumbler (there are no such restrictions in Canadian football). However, if the offense fumbles the ball, the defense recovers and then fumbles back to the offense, they would get a first down since possession had formally changed over the course of the play even though the ball had never been blown dead. In American football, there is no separate signal to indicate a fumble recovery. If the offense recovers its own fumble, the official will indicate the recovery by a hand signal showing the next down. If the defense recovers the fumble, the official will indicate with a "first down" signal in the direction the recovering team is driving the ball. Some officials have erroneously used a "first down" signal when the offense recovers its own fumble and the recovery did not result in a first down.

...
Wikipedia