circus (n.)

late 14c., in reference to the large, oblong, unroofed enclosures used for races, etc., in ancient Rome, from Latin circus "ring, circular line," which was applied by Romans to circular arenas for performances and contests and oval courses for racing (especially the Circus Maximus), from or cognate with Greek kirkos "a circle, a ring," perhaps from PIE *kikro-, reduplicated form of root *sker- (2) "to turn, bend." The adjective form is circensian.

In reference to modern large arenas for performances of feats of horsemanship, acrobatics, etc., from 1791, sense then extended to the performing company itself and the entertainment given, hence "traveling show" (originally traveling circus, 1838). Extended in World War I to squadrons of military aircraft. Meaning "lively uproar, chaotic hubbub" is from 1869.

Sense in Picadilly Circus and other place names is from early 18c. sense "buildings arranged in a ring," also "circular road."

Definitions of circus
circus (n.)
a travelling company of entertainers; including trained animals;
he ran away from home to join the circus
circus (n.)
a performance given by a traveling company of acrobats, clowns, and trained animals;
the children always love to go to the circus
circus (n.)
a frenetic disorganized (and often comic) disturbance suggestive of a large public entertainment;
it was so funny it was a circus
Synonyms: carnival
circus (n.)
(antiquity) an open-air stadium for chariot races and gladiatorial games;
circus (n.)
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
Circus (n.)
a genus of haws comprising the harriers;
Synonyms: genus Circus