big top vs circus tent

big top and circus tent both are nouns.

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

As nouns, big top and circus tent are synonyms defined as:

big top and circus tent: a canvas tent to house the audience at a circus performance

Other synonyms of big top include round top, top.

big top (noun)	circus tent (noun)
a canvas tent to house the audience at a circus performance	a canvas tent to house the audience at a circus performance