Deism is the philosophical position and rationalistic theology that generally rejects revelation as a source of divine knowledge and asserts that empirical reason and observation of the natural world are exclusively logical, reliable, and sufficient to determine the existence of a Supreme Being as the creator of the universe. More simply stated, Deism is the belief in the existence of God, solely based on rational thought without any reliance on revealed religions or religious authority. Deism emphasizes the concept of natural theology—that is, God's existence is revealed through nature.

Deus is the Latin word for "god" or "deity". Latin deus and dīvus ("divine") are in turn descended from Proto-Indo-European *deiwos, "celestial" or "shining", from the same root as *Dyēus, the reconstructed chief god of the Proto-Indo-European pantheon.