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):

Colosseum
    n 1: a large amphitheater in Rome whose construction was begun
         by Vespasian about AD 75 or 80 [syn: Colosseum,
         Amphitheatrum Flavium]