A turnover in sports is a play in which one team unintentionally gives possession to the opposing team. In many sports, how often this happens to a team is thought to measure offensive carelessness, defensive excellence, or both. American football is probably the sport in which the most weight is given to this statistic. In football, it is either an interception — a pass thrown by the offense that is caught by the defense — or a fumble by the offense that is recovered by the defense. There are other plays in which possession changes hands, such as kickoffs, punts, and unconverted fourth down plays, but only fumbles and interceptions qualify as turnovers.
Statistics have long shown a strong correlation between turnovers and success in wins and losses. The team that turns the ball over less in a given game is significantly more likely to win that game, and therefore a large emphasis is placed on both protecting the ball on offense and forcing fumbles and interceptions when on defense. Interceptions are calculated into the formula that is used to evaluate quarterbacks, and runners are often judged poorly if they are prone to fumbling. The turnover margin, a statistic which compares the number of turnovers created to the number committed, is commonly used to show the overall effectiveness of a team's offense and defense.
A turnover in basketball can occur when a player passes the ball to the opposing team or commits an offensive foul or some type of violation that gives the ball to the other team. In basketball, like football, it is a statistic that is given a lot of weight both for teams and for individual players. For a point guard, the player who handles the ball most frequently and who is assigned the task of distributing the ball and running the offense, the ratio of turnovers to assists — an assist is recorded when the player passes the ball to another player who then scores — is often used to show efficiency, or lack thereof.
In most sports, though, the term is used even when it is not kept as a statistic. In hockey or soccer, for example, it can be committed when a player loses control of the puck or ball and it is then gained by an opposing player. It doesn't have the significance it has in football or basketball, though, as the fluid structure of hockey and soccer allows for a consistent flow of possession from one team to the other.