ˈfoul line noun [countable]
a line marked on a sports field outside of which a ball cannot be legally played

[TahlilGaran] Dictionary of Contemporary English ▲

foul line
n.
1. Either of two lines separating fair from foul ground in baseball.
Willie hit the ball just inside the foul line for a double.
2. A line across the upper end of a bowling alley across which a bowler must not step.
John bowled a strike but it didn't count because he stepped over the foul line.
3. A line on the floor in front of the basket in basketball, from which foul shots are made.
Tony scored eight points from the foul line.

[TahlilGaran] English Idioms Dictionary ▲