A fumble in American and Canadian football is any act other than passing, kicking 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 two events considered to be turnovers, where possession of the ball can change during play.

Technically, under American rules if a player drops the ball while attempting to catch a lateral pass it is a muff (you can't "fumble" a loose ball). The result is the same and most announcers will still call it a fumble. Muffs also result when the ball is improperly fielded on kicking plays such as punts. Ball security is a term used to describe the ability of a player to maintain control over the football during play and thus avoid a fumble.

In American football, The Fumble refers to a specific incident in the AFC Championship Game between the Cleveland Browns and the Denver Broncos on January 17, 1988 at Mile High Stadium. With 1:12 left in the game, running back Earnest Byner appeared to be on his way to score the game-tying touchdown, but lost a fumble at the 3-yard line.