Avestan

[uh-ves-tuh n] /əˈvɛs tən/
noun
1.
an ancient East Iranian language of the Indo-European family, the language of all the Avesta but the Gathas.
Compare Gathic (def 1).
adjective
2.
of or pertaining to the Avesta or its language.
Origin
1855-60; Avest(a) + -an
British Dictionary definitions for Avestan

Avestan

/əˈvɛstən/
noun
1.
the oldest recorded language of the Iranian branch of the Indo-European family; the language of the Avesta Formerly called Zend
adjective
2.
of or relating to the Avesta or its language
Word Origin and History for Avestan

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."