The Collaborative International Dictionary of English v.0.48:

Colosseum \Col`os*se"um\, n. [Neut., fr. L. colosseus gigantic.
   See Coliseum.]
   The amphitheater of Vespasian in Rome. [Also written
   Coliseum.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Coliseum \Col`i*se"um\, n. [NL. (cf. It. coliseo, colosseo), fr.
   L. colosseus colossal, fr. colossus a colossus. See
   Colossus, and cf. Colosseum.]
   The amphitheater of Vespasian at Rome, the largest in the
   world. [Written also Colosseum.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

coliseum
    n 1: an oval large stadium with tiers of seats; an arena in
         which contests and spectacles are held [syn:
         amphitheater, amphitheatre, coliseum]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "coliseum":
   agora, amphitheater, arena, athletic field, auditorium, background,
   bear garden, bowl, boxing ring, bull ring, campus, canvas, circus,
   cockpit, colosseum, course, field, floor, forum, ground, gym,
   gymnasium, hall, hippodrome, lists, locale, marketplace, mat,
   milieu, open forum, palaestra, parade ground, pit, place, platform,
   precinct, prize ring, public square, purlieu, range, ring, scene,
   scene of action, scenery, setting, site, sphere, squared circle,
   stadium, stage, stage set, stage setting, terrain, theater,
   tilting ground, tiltyard, walk, wrestling ring