Entry 1 sense Webster, 1913 Monotheism /män'-əth-ēˌ-ĭz-əm/ · Mon·o·the·ism · IPA /ˌmɑnoʊ̯ˈθiɪzm̩/ 01 n. The doctrine or belief that there is but one God. 1. The doctrine or belief that there is but one God.