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. Unlike other events which cause the ball to become loose, such as an incomplete pass, a fumbled ball is considered a live ball, and may be recovered and advanced by any member of either team.

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.

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 they simply try 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 a half/overtime unless the fumbler recovers the ball (there are no such restrictions in Canadian football). However, the defense cannot advance the ball either if they fumble the ball back to the offense (unless it is recovered by the defensive fumbler). However, if the offense fumbles the ball, the defense recovers, and then fumbles back to the offense, they would get a new 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 and down it, 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.

It is not a fumble when a forward pass is attempted and is not caught. In this latter case, it is simply an incomplete pass. However, if the receiver catches the ball, but then drops it after gaining control of the ball, that is a fumble.

Any number of fumbles can be committed during a play, including fumbles by the team originally on defense. Most famously, Dallas Cowboys defender Leon Lett fumbled during Super Bowl XXVII while celebrating during his own fumble return.

A sometimes controversial maxim is "the ground cannot cause a fumble". If a player is tackled and loses control of the ball at or after the time he makes contact with the ground, the player is treated as down and the ball is not in play. However, in the NFL and CFL, if a ball carrier falls without an opponent contacting him, the ground can indeed cause a fumble. This is because in those leagues the ball carrier is not "down" unless an opponent first makes contact, or the runner is out of bounds. In most other leagues, as soon as the knee or elbow touches the ground, the ball carrier is considered down. It is also possible for the ground to cause a fumble in college football if the ball hits the ground before any part of the ball carrier's body (other than the hand or foot) touches the ground. An example was the fumble by Arkansas quarterback Clint Stoerner vs. Tennessee in 1998.