Source : Webster's Revised Unabridged Dictionary (1913)

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}.]

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

Source : WordNet®

Colosseum
     n : amphitheater in Rome built about AD 75 or 80