twilight has 10 distinct synonyms spread across 2 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.
the diffused light from the sky when the sun is below the horizon but its rays are refracted by the atmosphere of the earth
a condition of decline following successes
“in the twilight of the empire”
Synonyms, antonyms and the broader and narrower relations for twilight come from Princeton University’s WordNet. Full sources and licences.