Wikipedia's definition of a Circus

A circus is commonly a travelling company of performers that may include clowns, acrobats, trained animals, trapeze acts, musicians, hoopers, tightrope walkers, jugglers, unicyclists and other stunt-oriented artists. The word also describes the performance that they give, which is usually a series of acts choreographed to music and introduced by a ringmaster. A traditional circus performance is normally held in a ring 13 m (42 ft) in diameter. This dimension was adopted by Philip Astley to enable a horse rider to stand upright on a cantering horse to perform a series of acrobatic maneuvers and to more easily retain their balance. Most modern circuses have a system of tiered seating around the ring for the public and since the late 19th early 20th century the performance has taken place under canvas and more recently plastic tents commonly called "The Big Top" .

Origin

In Ancient Rome, the circus was a building for the exhibition of horse and chariot races, equestrian shows, staged battles, displays featuring trained animals, jugglers and acrobats. The circus of Rome is thought to have been influenced by the Greeks, with chariot racing and the exhibition of animals as traditional attractions. The Roman circus consisted of tiers of seats running parallel with the sides of the course, and forming a crescent around one of the ends. The lower seats were reserved for persons of rank; there were also various state boxes, e.g. for the giver of the games and his friends. In Ancient Rome the circus was the only public spectacle at which men and women were not separated.

The first circus in Rome was the Circus Maximus, in the valley between the Palatine and Aventine hills. It was constructed during the monarchy and, at first, built completely from wood. After being rebuilt several times, the final version of the Circus Maximus could seat 250,000 people; it was built of stone and measured 400m in length and 90m in width.  Next in importance to the Circus Maximus in Rome were the Circus Flaminius and the Circus Neronis, from the notoriety which it obtained through the Circensian pleasures of Nero. A fourth, the Circus of Maxentius, was constructed by Maxentius; the ruins of this circus have helped archaeologists to reconstruct the Roman circus.

For some time after the fall of Rome, Europe lacked a large and animal-rich circus. Itinerant showmen travelled the fairgrounds of Europe. Animal trainers and performers are thought to have exploited the nostalgia for the Roman circus, travelling between towns and performing at local fairs. Another possible link between the Roman and modern circus could have been bands of Gypsies who appeared in Europe in the 14th century and in Britain from the 15th century, bringing with them circus skills and trained animals.