Zend

Webster's Dictionary

(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

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.