big top vs circus

big top and circus both are nouns.

Word	Noun	Adjective	Verb	Adverb
big top	Yes	No	No	No
circus	Yes	No	No	No

big top (noun)	circus (noun)
a canvas tent to house the audience at a circus performance	a performance given by a traveling company of acrobats, clowns, and trained animals
	a frenetic disorganized (and often comic) disturbance suggestive of a large public entertainment
	an arena consisting of an oval or circular area enclosed by tiers of seats and usually covered by a tent
	(antiquity) an open-air stadium for chariot races and gladiatorial games
	a travelling company of entertainers; including trained animals