The Collaborative International Dictionary of English v.0.48:

Ringmaster \Ring"mas`ter\, n.
   One in charge of the performances (as of horses) within the
   ring in a circus.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ringmaster
    n 1: the person in charge of performances in a circus ring

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "ringmaster":
   MC, acrobat, aerialist, auteur, ballyhoo man, bareback rider,
   barker, callboy, circus artist, clown, costume designer, costumer,
   costumier, director, emcee, equestrian director, equilibrist,
   exhibitor, flier, high-wire artist, impresario, juggler,
   lion tamer, makeup man, master of ceremonies, playreader, producer,
   prompter, scenewright, set designer, showman, slack-rope artist,
   snake charmer, spieler, stage director, stage manager,
   sword swallower, theater man, theatrician, ticket collector,
   tightrope walker, trapeze artist, tumbler, usher, usherer,
   usherette