Definition of foul line:
(n) :
(baseball, softball) One of the two lines on the edge of a field marking the area inside which a fair ball may be hit.
(n) :
(basketball) A line from which a player takes a free throw
(n) :
(bowling) A line between the lane and the approach, which the bowler must not cross.
(n) :
(athletics) In long jump, a line before which the jumper must start their jump with no part of the foot touching the line.