circus has 2 distinct synonyms spread across 2 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.
a travelling company of entertainers; including trained animals
“he ran away from home to join the circus”
a performance given by a traveling company of acrobats, clowns, and trained animals
“the children always love to go to the circus”
a frenetic disorganized (and often comic) disturbance suggestive of a large public entertainment
“it was so funny it was a circus”
(antiquity) an open-air stadium for chariot races and gladiatorial games
an arena consisting of an oval or circular area enclosed by tiers of seats and usually covered by a tent
“they used the elephants to help put up the circus”
Synonyms, antonyms and the broader and narrower relations for circus come from Princeton University’s WordNet. Full sources and licences.