Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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