The Collaborative International Dictionary of English v.0.48:

Polygamy \Po*lyg"a*my\, n. [Gr. ?; cf. F. polygamie.]
   1. The having of a plurality of wives or husbands at the same
      time; usually, the marriage of a man to more than one
      woman, or the practice of having several wives, at the
      same time; -- opposed to monogamy; as, the nations of the
      East practiced polygamy. See the Note under Bigamy, and
      cf. Polyandry.
      [1913 Webster]

   2. (Zool.) The state or habit of having more than one mate.
      [1913 Webster]

   3. (Bot.) The condition or state of a plant which bears both
      perfect and unisexual flowers.
      [1913 Webster]

WordNet (r) 3.0 (2006):

polygamy
    n 1: having more than one spouse at a time

The Devil's Dictionary (1881-1906):

POLYGAMY, n.  A house of atonement, or expiatory chapel, fitted with
several stools of repentance, as distinguished from monogamy, which
has but one.