The Collaborative International Dictionary of English v.0.48:

Avesta \A*ves"ta\, n.
   The Zoroastrian scriptures; the sacred text of
   Zoroastrianism. See Zend-Avesta.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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