The Collaborative International Dictionary of English v.0.48:

Monophonic \Mon`o*phon"ic\, a. [Mono- + Gr. ? a voice.]
   1. (Mus.) Single-voiced; having but one part; as, a
      monophonic composition; -- opposed to polyphonic.
      [1913 Webster]

   2. Of or relating to a system for recording and reproducing
      sound, which has only one sound channel; also called
      monaural or mono. It contrasts with stereophonic (or
      stereo), quadraphonic, or surround-sound, which have
      two or more channels, and can thus reproduce the effect of
      the sound coming from more than one direction.
      [PJC]

WordNet (r) 3.0 (2006):

monophonic
    adj 1: designating sound transmission or recording or
           reproduction over a single channel [syn: mono,
           monophonic, single-channel]
    2: consisting of a single melodic line [ant: contrapuntal,
       polyphonic]