Definition of foul line
a line marking the boundary of permissible movement or play, in particular. Baseball either of the straight lines extending from home plate past first and third bases into the outfield and marking the limit of the area within which a hit is deemed to be fair. Basketball either of the lines 15 feet in front of each backboard, from which free throws are made. Also called free-throw line. (in bowling) a line on the alley, perpendicular to the gutters and 60 feet from the head pin.