Source : Webster's Revised Unabridged Dictionary (1913)

Amphitheater \Am`phi*the"a*ter\, Amphitheatre
\Am`phi*the"a*tre\,, n. [L. amphitheatrum, fr. Gr. ?; ? + ?
   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.

   Note: The Romans first constructed amphitheaters for combats
         of gladiators and wild beasts.

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

Source : WordNet®

amphitheatre
     n 1: a sloping gallery with seats for spectators (as in an
          operating room or theater) [syn: {amphitheater}]
     2: an oval large stadium with tiers of seats; an arena in which
        contests and spectacles are held [syn: {amphitheater}, {coliseum}]