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):

amphitheatre
    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]