Sasanian empire in the context of "House of Sasan"

Avestan (/əˈvɛstən/ ə-VESS-tən) is the liturgical language of Zoroastrianism. It belongs to the Iranian branch of the Indo-European language family and was originally spoken during the Avestan period (c. 1500 – 400 BCE) by the Iranians living in eastern Greater Iran as evidenced from names in Avestan geography.

After Avestan became extinct, its religious texts were transmitted orally, then collected and put into writing during the Sasanian period (c. 400 – 500 CE). The extant material falls into two groups: Old Avestan (c. 1500 – 900 BCE) and Younger Avestan (c. 900 – 400 BCE). The immediate ancestor of Old Avestan was the Proto-Iranian language, a sister language to the Proto-Indo-Aryan language, with both having developed from the earlier Proto-Indo-Iranian language. As such, Old Avestan is quite close in both grammar and lexicon to Vedic Sanskrit, the oldest preserved Indo-Aryan language.

The Sasanian Avesta or Great Avesta refers to the anthology of Zoroastrian literature produced during the Sasanian period. They are said to have been written down during the reign of Khosrow. Most of this work is now lost, but its content and structure can be reconstructed from references found in a number of texts from the 9th century onward.

Compared to the extant Avesta, the Sasanian Avesta was much larger and organized into 21 distinct volumes called nasks (Avestan: naska; Middle Persian: nask, 'bundle'). Of those, only one is preserved in its entirety, while others are either lost or only preserved in fragments.