[foul line] n (1878) 1: either of two straight lines extending from the rear corner of home plate through the outer corners of first and third base respectively and prolonged to the boundary of a baseball field
2: a line across a bowling alley that a player must not step over when delivering the ball
3: either of two lines on a basketball court parallel to and 15 feet from the backboards behind which a player must stand while shooting a free throw