WordNet records no synonym and no antonym for foul line. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a line from which basketball players take penalty shots
a line across a bowling alley that a bowler must not cross
lines through 1st and 3rd base indicating the boundaries of a baseball field
Synonyms, antonyms and the broader and narrower relations for foul line come from Princeton University’s WordNet. Full sources and licences.