Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

polytheism
     n : belief in multiple Gods [ant: {monotheism}]