big top 
noun
The main tent in a circus.He may be busy signing up circuits all over the world and encouraging new teams to enter the sport, but it would count for little if the main acts quit his big top for a rival circus....
- With the Chinese State Circus setting up its big top on Blackheath, reporter Michelle Frame decided it was time to stop clowning around in the office and paid the world-famous troupe a visit…
- The curtain rises earlier than normal with edge of your seat entertainment courtesy of Swamp Circus in a festival big top on the Glebe, Bowness, from Thursday, August 21 until Monday, August 25.