Monotheistic refers to a religion that follows monotheism.
Monotheism is defined as belief in the existence of one god or in the oneness of God. The Oxford Dictionary of the Christian Church gives a more restricted definition: "belief in one personal and transcendent God", as opposed to polytheism and pantheism. A distinction may be made between exclusive monotheism, and both inclusive monotheism and pluriform monotheism which, while recognising many distinct gods, postulate some underlying unity.
The word monotheism comes from the Greek μόνος (monos) meaning "single" and θεός (theos) meaning "god".