The Collaborative International Dictionary of English v.0.48:

Zend-Avesta \Zend`-A*ves"ta\, n. [Properly, the Avesta, or
   sacred text, and its zend, or interpretation, in a more
   modern and intelligible language. --W. D. Whitney.]
   The sacred writings of the ancient Persian religion,
   attributed to Zoroaster, but chiefly of a later date.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Zend-Avesta
    n 1: a collection of Zoroastrian texts gathered during the 4th
         or 6th centuries [syn: Avesta, Zend-Avesta]