From Longman Dictionary of Contemporary Englishbig topˌbig ˈtop noun [countable] 
DLOthe very large tent in which a circus performance takes place
DLOthe very large tent in which a circus performance takes placeExamples from the Corpus
big top• I put it on under a big top in 1967, not far from Ramatuelle on the Cte d'Azur.• Market traders were put out when it was announced they would be losing their pitches to accommodate the big top.• The activity centres around the big top in Stockton High Street offering free all day entertainment with live broadcasts and personal appearances.• Yet no one walked out of the big top, there were no fundamentalist pickets outside, no attempts to disrupt the service. 