A fumble in gridiron football occurs when a player who has possession and control of the ball loses it before being downed (tackled), scoring, or going out of bounds. By rule, it is any act other than passing, kicking, punting, or successful handing that results in loss of ball possession by a player.
A fumble may be forced by a defensive player who either grabs or punches the ball or butts the ball with their 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/overtime or on 4th down at any point during the game, when the fumbler is the only offensive player allowed to advance the ball, otherwise the ball is ruled dead at the spot of the fumble, except when it is recovered for a loss).
A fumble is one of three events that can cause a turnover (the other two being an interception or a turnover on downs).
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 clear possession of, such as while attempting to catch a lateral pass or improperly fielding a kicking play such as a punt (a player 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. Thus, losing possession of the ball via a fumble includes not only dropping the ball before being downed; but, also having a ball taken away, or “stripped” from the runner's possession before being downed.