A penalty called when a defensive player crosses the line of scrimmage prior to the snap of the football by the offense. Offsides result in a five-yard penalty. Offsides can also be called on the kicking team during a kickoff if they cross the line before the kicker makes contact with the ball. A defensive player is allowed to return to his side without penalty as long as the ball is not snapped, he does not make contact with the opposing team, and his actions do not create a false start by the offensive team.
A defensive player may end up offside by attempting to gain an advantage on the offense. Pass rushers will attempt to time the snap count, and they can end up offside if they time it poorly or get too excited to start the play.
A player may also be drawn offside by the opposing quarterback. Quarterbacks may use a fake cadence, sometimes referred to as a "hard count", to lure the defensive player into coming offside prior to the snap. If the defensive player does not make contact with an offensive player and the ball is snapped, the offensive team gets to complete the play and then decide between the result of the play or the five-yard penalty. Offensive teams will often use this free play to complete a big gain, knowing that they can always accept the five-yard penalty.