Avesta is the oldest extant Iranian language. It belongs to the Indo-Iranian family of languages. It is the mother of other Iranian languages like Old Persian, Middle Persian, Kurdish, Pashtu and Ossetic. Avesta heads the Iranian branch of Indo-Iranian language, just as Vedic Sanskrit is the source for the Indian branch, which has languages like Hindustani, Bengali and Marathi. The striking similarity between Vedic Sanskrit and Avestan is on account of their common origin.
English-Avesta Dictionary by K.E. Kanga (avesta.org)