In American football, a turnover occurs when the team with the ball loses possession of the ball, which is then gained by the other team. The two events that are officially classified as "turnovers" are fumbles (accidental physical loss of a live ball a player has possession of) or interceptions (passes intended for a member of the passing team, but caught by a member of the opposing team).
The term "turnover" is also used to refer to a turnover on downs, when a team attempts to gain a first down or touchdown on a fourth down play (known as a fourth down conversion), but is unsuccessful. When this occurs, the opposing team automatically gains possession at the spot to which the ball was advanced at the end of the play, unless a penalty has occurred that would change this outcome. The team that has lost possession is not permitted any opportunity to advance the ball any further.