ancient Persian god of light or the sun, eventually regarded as ruler of the material and spiritual universe, 1550s, from Latin, from Greek Mithras, from Avestan Mithra-, from Indo-Iranian *mitram "contract," whence *mitras "contractual partner, friend," conceptualized as a god, or, according to Kent, first the epithet of a divinity and eventually his name. Perhaps from PIE root *mei- (1) "to change; exchange," on the notion of "god of the contract" [Watkins].
Related to Sanskrit Mitrah, a Vedic deity associated with Varuna. "His name is one of the earliest Indic words we possess, being found in clay tablets from Anatolia dating to about 1500 B.C." [Calvert Watkins, "Dictionary of Indo-European Roots," 2000]. His worship was adopted by the Romans and enjoyed great popularity in the early empire. Related: Mithraic; Mithraism.