From BiblePortal Wikipedia

Webster's Dictionary [1]

(n.) Properly, the translation and exposition in the Huzv/resh, or literary Pehlevi, language, of the Avesta, the Zoroastrian sacred writings; as commonly used, the language (an ancient Persian dialect) in which the Avesta is written.

The Nuttall Encyclopedia [2]

Name applied, mistakenly it would seem, by the Europeans to the ancient Iranian language of Persia, or the language in which the Zend-Avesta is written, closely related to the Sanskrit of the Vedas it appears.