The Collaborative International Dictionary of English v.0.48:

Polygamous \Po*lyg"a*mous\, a. [Gr. ? living ? polygamy; poly`s
   many + ? marriage. Cf. Bigamy.]
   [1913 Webster]
   1. Of or pertaining to polygamy; characterized by, or
      involving, polygamy; having a plurality of wives; as,
      polygamous marriages; -- opposed to monogamous.
      [1913 Webster]

   2. (Zool.) Pairing with more than one female.
      [1913 Webster]

            Most deer, cattle, and sheep are polygamous.
                                                  --Darwin.
      [1913 Webster]

   3. (Bot.) Belonging to the Polygamia; bearing both
      hermaphrodite and unisexual flowers on the same plant.
      [1913 Webster]

WordNet (r) 3.0 (2006):

polygamous
    adj 1: having more than one mate at a time; used of
           relationships and individuals [ant: monogamous]
    2: having several forms of gametoecia on the same plant [syn:
       heteroicous, polyoicous, polygamous]