What it is

A player is offside when he is across the line of scrimmage, in the neutral zone, at the moment the ball is snapped. The foul is usually called on the defense.

In a game

The penalty is five yards. Defenders try to time the snap and fire off the line, and one who jumps early may have been fooled by a hard count, a snap count with an unusual rhythm. On kickoffs the kicking team can also be called for being offside.

More detail

Encroachment is a related foul. The defender crosses the line and touches an opponent before the snap. Officials and offenses watch the neutral zone, the space between the two ends of the ball, closely, because a defender who crosses it can also draw a false start from an offensive player who flinches.

Animated diagram

1. Both teams line up on their own side of the line of scrimmage, the neutral zone between them.
2. Before the ball is snapped, a defender steps across the line.
3. He is offside, the officials throw a flag and the penalty is five yards.