Avestan

an ancient East Iranian language of the Indo-European family, the language of all the Avesta but the Gathas.
Compare Gathic (def 1).
of or relating to the Avesta or its language.
Historical Examples

The Avestan Morals are brought out by Mr. Johnson in their original and exceeding purity.
The Bay State Monthly, Volume 3, No. 2 Various

The reciprocal help of the deity and the king (19–21) is Avestan, and inconsistent with monotheism.
Solomon and Solomonic Literature Moncure Daniel Conway

The language of the Avesta can be correctly describedonly as Avestan, for no other literature in the same language exists.
The Worlds Greatest Books, Volume XIII. Various

Among the Avestan Fragments, attention might finally be called to one which we must be glad has not been lost.
Library Of The World’s Best Literature, Ancient And Modern, Vol 3 Various

noun
the oldest recorded language of the Iranian branch of the Indo-European family; the language of the Avesta Formerly called Zend
adjective
of or relating to the Avesta or its language

Eastern Iranian language that survived in sacred texts centuries after it went extinct, from Persian Avesta “sacred books of the Parsees,” earlier Avistak, literally “books.”