Football an imaginary line, parallel to the goal lines, on which the ball rests at the start of each play and on either side of which the teams line up
See line of scrimmage in American Heritage Dictionary 4
nounpl.lines of scrimmage Football
Either of two imaginary lines extending across the field parallel to the goal line at the ends of the ball as it rests prior to being snapped and at which each team lines up for a new play.