An offside in football is a foul usually committed by a player on the defending team. Offsides is a very common penalty in football. It occurs when any part of the player crosses the line of scrimmage and is within the neutral zone when the ball is snapped. When the defender is called offsides, a 5 yard penalty will be assessed, resulting is a gain of 5 yards for the offense.
Play is not stopped for an offside penalty, so many consider it to be a free play for the offense. If the resulting play gains significant yardage then the team may choose to decline the penalty and advance. On the other hand, if the play does not gain yardage then the offense can accept the penalty and gain 5 yards.
Similarly, if an offensive player crosses the line of scrimmage during the snap, they will be charged with a false start penalty and play will stop. Offside penalties will be given at most levels of football including NCAA and NFL football. Other comparable penalties include a neutral zone infraction, a false start, or encroachment.