Deus (pronounced 1) is the Latin word for "god" or "deity". The Latin words deus and dīvus, and Greek διϝος = "divine", are descended from Proto-Indo-European *deiwos = "divine", from the same root as Dyēus, the reconstructed chief god of the Proto-Indo-European pantheon, also a cognate of the Greek Ζευς (Zeus). By the era of Classical Latin it was a general noun referring to any number of divine figures. The word continues to refer directly to God in the Portuguese language. It is also incorporated into a number of phrases and slogans. For example, nobiscum deus ("God with us") was a battle cry of the late Roman Empire and of the Byzantine Empire.

Dei is an inflected form of deus, used in such phrases as Roman Catholic organization Opus Dei (work of God), Agnus Dei (Lamb of God) and Dei Gratia (By the Grace of God). It is most often the genitive case ("of god"), but is also the primary plural form in addition to the variant di. There is another plural sometimes used, dii, and a feminine form deae ("goddesses").

The word "Deus," through "Dei," is the root of deism, pandeism, panendeism, and polydeism, ironically all of which are theories in which any divine figure is absent from intervening in human affairs. This curious circumstance originates from the use of the word "deism" in the 17th and 18th centuries as a contrast to the prevailing "theism", belief in an actively intervening God:

Followers of these theories, and occasionally followers of pantheism, may sometimes refer to God as "Deus" or "the Deus" to make clear that the entity being discussed is not a theistic "God". Arthur C. Clarke used this in his novel, 3001: The Final Odyssey. There, the term Deus has replaced God in the 31st Century, the word God being associated with religious fanaticism. The prevailing religious view in Clarke's story is Deism.

St. Jerome translated the Hebrew word Elohim (אֱלוֹהִים , אלהים) into Latin as Deus.

Some uses of the word have negative connotations. In Cartesian philosophy, the phrase deus deceptor is sometimes used to discuss the possibility of an evil God that seeks to deceive us. This character is related to a skeptical argument as to how much we can really know if an evil daemon were attempting to thwart our knowledge. Another is the deus otiosus ("idle god"), a theological concept used to describe the belief in a creator god who largely retires from the world and is no longer involved in its daily operation. A similar concept is that of the deus absconditus ("hidden god") of Thomas Aquinas. Both refer to a deity whose existence is not readily knowable by humans through either contemplation or examination of divine actions. The concept of deus otiosus often suggests a god who has grown weary from involvement in this world and who has been replaced by younger, more active gods, whereas deus absconditus suggests a god who has consciously left this world to hide elsewhere.