The Avesta is the principal surviving scripture of Zoroastrianism. It preserves prayers, hymns, ritual instructions, religious laws, and other texts associated with the Zoroastrian tradition of ancient Iran.
The collection was transmitted orally for centuries before being written down and compiled in later periods. Much of the original material has been lost, so the surviving Avesta represents only part of the larger religious literature that once existed. Its oldest section is generally considered to be the Gathas, hymns traditionally attributed to Zoroaster.
“Zend-Avesta” is a historical expression, but it can cause confusion: Zend properly refers to commentary or interpretation rather than being the name of a separate scripture. The Avesta also should not be treated as a single book written at one time; it is a compilation of texts from different periods and genres.