- the doctrine or belief that there is only one God.
- the belief or doctrine that there is only one God
n.1670s, from monotheism + -ist. n.“belief that there is but one god,” 1650s, from Greek mono-, comb. form of monos “single, alone” (see mono-) + theos “a god” (see Thea). A belief in one god. Judaism, Christianity, and Islam are all monotheistic religions. (Compare polytheism.)