interval has 3 distinct synonyms spread across 3 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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 definite length of time marked off by two instants
a set containing all points (or all real numbers) between two given endpoints
Synonyms, antonyms and the broader and narrower relations for interval come from Princeton University’s WordNet. Full sources and licences.