The Collaborative International Dictionary of English v.0.48:

Circus \Cir"cus\, n.; pl. Circuses. [L. circus circle, ring,
   circus (in sense 1). See Circle, and cf. Cirque.]
   [1913 Webster]
   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.
      [1913 Webster]

   Note: The Circus Maximus at Rome could contain more than
         100,000 spectators. --Harpers' Latin Dict.
         [1913 Webster]

   2. A circular inclosure for the exhibition of feats of
      horsemanship, acrobatic displays, etc. Also, the company
      of performers, with their equipage.
      [1913 Webster]

   3. Circuit; space; inclosure. [R.]
      [1913 Webster]

            The narrow circus of my dungeon wall. --Byron.
      [1913 Webster]

WordNet (r) 3.0 (2006):

circus
    n 1: a travelling company of entertainers; including trained
         animals; "he ran away from home to join the circus"
    2: a performance given by a traveling company of acrobats,
       clowns, and trained animals; "the children always love to go
       to the circus"
    3: a frenetic disorganized (and often comic) disturbance
       suggestive of a large public entertainment; "it was so funny
       it was a circus"; "the whole occasion had a carnival
       atmosphere" [syn: circus, carnival]
    4: (antiquity) an open-air stadium for chariot races and
       gladiatorial games
    5: 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"
    6: a genus of haws comprising the harriers [syn: Circus,
       genus Circus]

Moby Thesaurus II by Grady Ward, 1.0:

149 Moby Thesaurus words for "circus":
   Broadway, O, agora, amphitheater, annular muscle, annulus, arena,
   areola, athletic field, auditorium, aureole, background,
   bear garden, bowl, boxing ring, bull ring, burlesque,
   burlesque show, campus, canvas, carnival, chaplet, circle, circuit,
   circumference, closed circle, cockpit, coliseum, colosseum,
   coochie show, corona, coronet, course, crescent, crown, cycle,
   diadem, discus, disk, drama, entertainment industry,
   eternal return, fairy ring, fantoccini, field, floor, floor show,
   forum, galanty show, garland, girly show, glory, ground, gym,
   gymnasium, hall, halo, hippodrome, hootchy-kootchy show, lasso,
   leg show, legit, legitimate stage, light show, lists, locale,
   logical circle, loop, looplet, magic circle, magic show,
   marketplace, mat, milieu, noose, off Broadway, off-off-Broadway,
   ombres chinoises, open forum, orbit, palaestra, parade ground,
   peep show, pit, place, platform, playland, precinct, prize ring,
   public square, puppet show, purlieu, radius, range, raree-show,
   rep show, repertory drama, repertory show, ring, rodeo, rondelle,
   round, roundel, saucer, scene, scene of action, scenery, setting,
   shadow show, show biz, show business, sideshow, site, sphere,
   sphincter, squared circle, stadium, stage, stage set,
   stage setting, stage world, stagedom, stageland, stock, strawhat,
   strawhat circuit, summer stock, terrain, the big top, the boards,
   the footlights, the scenes, the stage, the theater, theater,
   theater world, theatromania, theatrophobia, tilting ground,
   tiltyard, variety, variety show, vaudeville, vaudeville show,
   vicious circle, walk, wheel, wreath, wrestling ring



The Devil's Dictionary (1881-1906):

CIRCUS, n.  A place where horses, ponies and elephants are permitted
to see men, women and children acting the fool.