The Collaborative International Dictionary of English v.0.48:

Odeon \O*de"on\, n. [NL., fr. Gr. ?, fr. ?: cf. F. od['e]on. See
   Ode.]
   A kind of theater in ancient Greece, smaller than the
   dramatic theater and roofed over, in which poets and
   musicians submitted their works to the approval of the
   public, and contended for prizes; -- hence, in modern usage,
   the name of a hall for musical or dramatic performances.
   [1913 Webster]