In American football
, a play that occurs near the end of the game when a team is in desperate need of the ball.
Kickoffs in football occur at the start of each half, and after each touchdown and field goal. The team who just scored kicks the ball from their own 30-yard line, and the other team receives the ball and may run it back as far as they are able.
However, the rules state that once the ball has gone ten yards from the kick, or once it has been touched by a member of the receiving team, the ball is fair game -- anyone may grab it and gain possession.
So when a team is in a position where they will lose the game if the other team gains possession of the ball, that is, they are behind with so little time left that the other team can simply run out the clock, they will try an onside kick, in which they try to kick the ball ten yards and then grab it, retaining possession.
What this looks like is ten players line up on the side of the field, and the kicker in the middle kicks the ball in a rolling kick towards the sidelines. The ten players then rush the towards the other team (who has assembled ten yards downfield) and hope that they will get there fast enough and be able to get the ball. The difficulty in this situation lies in the odd shape of the football; it's hard to tell how the ball is going to bounce, so recovering an onside kick usually requires some good luck.
The name of the play comes from the kicker placing the ball on its side rather than placing it upright for the kick. However, some kickers will place the ball on its stand for an onside kick, kicking the top of the ball and driving it into the turf, trying to bounce it high up in the air and making a sort of jump ball for his teammates.
Onside kicks are successful about 20% of the time in the NFL, and are typically only played as a last resort.