The line of scrimmage is an imaginary line that stretches across the width of an American football field, marking where each play begins. Before the ball is snapped, the offense lines up on one side of this line and the defense lines up on the other. Neither team can cross it until the play starts, or they'll be penalized for being offside.

The line isn't painted on the field. Instead, it runs through wherever the football is placed at the end of the previous play. When you watch a football game on TV, you'll often see a digital line drawn across the screen showing where the line of scrimmage is. In American football, the offense needs to advance the ball ten yards beyond this line within four plays (called downs) to keep possession of the ball.