Deiwos is the presumed Proto-Indo-European (PIE) word for god, the divine, and deity (*deiwo being the root). It was never directly a deity by itself, as the reconstructed PIE pantheon incorporated a variety of gods. The closest named god in the mythology was Dyeus Phater, who is the equivalent of Zeus and Jupiter in their respective pantheons.
These conclusions are drawn from linguistics and linguistic anthropology, specifically from witin the field of language reconstruction. Examples of the reconstuctive process include using the Greek for god: Zeus, or Zdeus as it may have been pronounced; the Latin for god, the divine, and deity: deus; and Sanskrit: diva, deva, dev; to draw constructive conclusions about the presumed root for the word in PIE.