The Collaborative International Dictionary of English v.0.48:

Polytheism \Pol"y*the*ism\, n. [Poly- + Gr. ? cf. F.
   polyth['e]isme.]
   The doctrine of, or belief in, a plurality of gods.
   [1913 Webster]

         In the Old Testament, the gradual development of
         polytheism from the primitive monotheism may be
         learned.                                 --Shaff-Herzog.
   [1913 Webster]

WordNet (r) 3.0 (2006):

polytheism
    n 1: belief in multiple Gods [ant: monotheism]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "polytheism":
   acosmism, allotheism, anthropolatry, anthropomorphism,
   anthropotheism, autotheism, cosmotheism, deism, ditheism, dualism,
   dyotheism, henotheism, hylotheism, monolatry, monotheism,
   multitheism, myriotheism, pantheism, physicomorphism, physitheism,
   psychotheism, tetratheism, theism, theopantism, theriotheism,
   tritheism, zootheism