The Collaborative International Dictionary of English v.0.48:

Amphitheater \Am`phi*the"a*ter\, Amphitheatre
\Am`phi*the"a*tre\,, n. [L. amphitheatrum, fr. Gr. ?; 'amfi` + ?
   theater: cf. F. amphith['e][^a]tre. See Theater.]
   1. An oval or circular building with rising tiers of seats
      about an open space called the arena.
      [1913 Webster]

   Note: The Romans first constructed amphitheaters for combats
         of gladiators and wild beasts.
         [1913 Webster]

   2. Anything resembling an amphitheater in form; as, a level
      surrounded by rising slopes or hills, or a rising gallery
      in a theater.
      [1913 Webster]

WordNet (r) 3.0 (2006):

amphitheater
    n 1: a sloping gallery with seats for spectators (as in an
         operating room or theater) [syn: amphitheater,
         amphitheatre]
    2: 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:

90 Moby Thesaurus words for "amphitheater":
   Elizabethan theater, Globe Theatre, Greek theater, agora, arena,
   arena theater, assembly hall, athletic field, auditorium,
   background, bear garden, bowl, boxing ring, bull ring, cabaret,
   campus, canvas, chapel, circle theater, circus, classroom, club,
   cockpit, coliseum, colosseum, concert hall, convention hall,
   course, dance hall, exhibition hall, field, floor, forum, gallery,
   ground, gym, gymnasium, hall, hippodrome, house, lecture hall,
   lecture room, lists, little theater, locale, marketplace, mat,
   meetinghouse, milieu, music hall, night spot, nightclub,
   open forum, opera, opera house, outdoor theater, palaestra,
   parade ground, pit, place, platform, playhouse, precinct,
   prize ring, public square, purlieu, range, recitation room, ring,
   scene, scene of action, scenery, schoolroom, setting, showboat,
   site, sphere, squared circle, stadium, stage, stage set,
   stage setting, terrain, theater, theater-in-the-round, theatron,
   tilting ground, tiltyard, walk, wrestling ring