The Collaborative International Dictionary of English v.0.48:

Polyphony \Po*lyph"o*ny\, n. [Gr. ?.]
   1. Multiplicity of sounds, as in the reverberations of an
      echo.
      [1913 Webster]

   2. Plurality of sounds and articulations expressed by the
      same vocal sign.
      [1913 Webster]

   3. (Mus.) Composition in mutually related, equally important
      parts which share the melody among them; contrapuntal
      composition; -- opposed to homophony, in which the melody
      is given to one part only, the others filling out the
      harmony. See Counterpoint.
      [1913 Webster]

WordNet (r) 3.0 (2006):

polyphony
    n 1: music arranged in parts for several voices or instruments
         [syn: polyphony, polyphonic music, concerted music]
         [ant: monody, monophonic music, monophony]