costume has 1 distinct synonym spread across 1 of its 6 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 attire worn in a play or at a fancy dress ball
“he won the prize for best costume”
unusual or period attire not characteristic of or appropriate to the time and place
“in spite of the heat he insisted on his woolen costume”
Synonyms, antonyms and the broader and narrower relations for costume come from Princeton University’s WordNet. Full sources and licences.
the prevalent fashion of dress (including accessories and hair style as well as garments)
the attire characteristic of a country or a time or a social class
“he wore his national costume”