In gridiron football, offside is a foul in which a player is on the wrong side of the line of scrimmage when the ball is snapped. This foul occurs simultaneously with the snap. Unlike offensive players, defensive players are not compelled to come to a set position before the snap. If a defender jumps across the line but gets back to his side before the snap, there is no foul. In the case of an offside foul, play is not stopped, and the foul is announced at the conclusion of the play. Media covering the games calls it a "free play" for the offense, as the non-offending team may decline the penalty and take the yardage gained on the play (and when the play works against them, like a turnover to the opposing team, the non-offending team can accept the penalty and retake possession of the ball) - unlike in the case of a false start foul against the offense, whereupon the play is immediately stopped by the officials.
This foul is almost always committed by the defense (any offensive player that moves into the neutral zone after setting would be charged with a false start). However, it is possible for the offense to commit this foul. If an offensive player lines up in the neutral zone, an offside foul will be called against the offense.
In the NFL and NCAA, the penalty for violation is 5 yards.