In American football, "scrimmage" refers to the phase of play that begins when the ball is snapped and moves until the play is whistled dead. It also designates the situation where teams practice with simultaneous offensive and defensive units. The line of scrimmage is critical, serving as the starting point for each play. Players must remain behind this line until the snap to avoid a penalty. Understanding scrimmage dynamics is essential for executing plays successfully and allows teams to strategize during gameplay.