The Collaborative International Dictionary of English v.0.48:

Monotheism \Mon"o*the*ism\, n. [Mono- + Gr. ? god: cf. F.
   monoth['e]isme.]
   The doctrine or belief that there is but one God.
   [1913 Webster]

WordNet (r) 3.0 (2006):

monotheism
    n 1: belief in a single God [ant: polytheism]

Moby Thesaurus II by Grady Ward, 1.0:

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