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]