Source: Webster's Revised Unabridged Dictionary (1913)Circus \Cir"cus\, n.; pl. Circuses. [L. circus circle, ring, circus (in sense 1). See Circle, and cf. Cirque.] 1. (Roman Antiq.) A level oblong space surrounded on three sides by seats of wood, earth, or stone, rising in tiers one above another, and divided lengthwise through the middle by a barrier around which the track or course was laid out. It was used for chariot races, games, and public shows.
Well in proper english (or Latin actually) it's circii. But i seem to think that circuses has been adopted in our modern english.
Atleast Latin is better than Greece
Actually in Latin it would be circi (just one i). Small, I know, but after four years of Latin in high school, I like to be able to use the knowledge sometimes. For anyone choosing a language to learn, don't pick Latin... it's not as much help as you might think.