Deus (pronounced 'deːus) is the Latin word for "god" or "deity". The Latin words deus and dīvus, and Greek διϝος = "divine", are believed to descend 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 Portuguese. But Eusebius Sophronius Hieronymus, also known as Saint Jerome translated the Hebrew word Elohim as used in the Bible (אֱלוֹהִים , אלהים) into Latin as Deus. One of the battle cries of the late Roman Empire and of the Byzantine Empire was Nobiscum deus ("God with us").
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 can also be a variant of the plural form, 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, theories in which any divine figure is, ironically, absent from intervening in human affairs. This curiousity originates from use of "deism" in the 17th and 18th centuries as a contrast to "theism", then the prevailing word for the belief in an actively intervening God:
Followers of these theories, and occasionally of pantheism, sometimes name God "Deus" or "the Deus" to make clear that the entity being discussed is not a theistic "God". Some uses of the word are negative. René Descartes used the phrase deus deceptor to discuss skeptic's argument that an evil God might deceives us. Another, deus otiosus ("idle god"), is a theological concept describing belief in a creator god who largely retires from the world and is no longer involved in its daily operation. Like the deus absconditus ("hidden god") of Thomas Aquinas, it refers to a deity with an existence is not readily knowable to humans through either contemplation or examination of divine actions. Deus otiosus suggests a god grown weary from involvement in this world, replaced by younger, more active gods (as in Sumerian and Greek myth), while deus absonditus suggests a god who consciously left the world to hide elsewhere.