An Etymological Dictionary of Astronomy and Astrophysics

The belief in one God as the creator and ruler of the universe, without rejection of revelation (distinguished from → deism).

2. Belief in the existence of a god or gods (opposed to → atheism) (Dictionary.com).

Etymology (EN): From the- variant of theo- before a vowel, from Gk. theos “god,” from PIE root *dhes-, root of words applied to various religious concepts, such as L. feriae “holidays,” festus “festive,” fanum “temple.”

Etymology (PE): Yzadân-bâvari, from yazdân “god,” from Mid.Pers. yazetân “gods,” ultimately from Proto-Ir. *iaz- “to sacrifice, worship, venerate,” → deity.