A fumble in American football and Canadian football occurs when an offensive player such as the quarterback or a running back drops the ball while it is still in play. A fumble may also be forced by a defensive player who either grabs or punches the ball or butts the ball with his helmet. Under a rules interpretation usually cited under the rubric "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 ball is treated as dead and not in play.

Fumbles usually occur during the snap, while running the ball, or in a failed attempt at a lateral pass. Technically, however, 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.

If the ball is fumbled the defensive team may recover the ball and even advance it to their goal. The same is true for the offense, but usually when the offense recovers the ball it simply tries to down it. The offense cannot advance the ball if it recovers its own fumble on fourth down, or in the last two minutes of the game. This is not the same thing as when a forward pass is attempted and the intended receiver does not catch it. In this latter case, it is simply an incomplete pass. However, if the receiver were to catch the ball, but then drop it after gaining control of the ball, that would be considered a fumble.