Avestan (), also known historically as Zend, comprises two languages: Old Avestan (spoken in the 2nd millennium BCE) and Younger Avestan (spoken in the 1st millennium BCE). The languages are known only from their use as the language of Zoroastrian scripture (the Avesta), from which they derive their name. Both are early Iranian languages, a branch of the Indo-Iranian languages within the Indo-European family. Its immediate ancestor was the Proto-Iranian language, a sister language to the Proto-Indo-Aryan language, with both having developed from the earlier Proto-Indo-Iranian. As such, Old Avestan is quite close in grammar and lexicon to Vedic Sanskrit, the oldest preserved Indo-Aryan language.
The Avestan text corpus was composed in ancient Arachosia, Aria, Bactria, and Margiana, corresponding to the entirety of present-day Afghanistan, and parts of Tajikistan, Turkmenistan, and Uzbekistan. The Yaz culture of Bactria-Margiana has been regarded as a likely archaeological reflection of the early "Eastern Iranian" culture described in the Avesta.
"Avestan, which is associated with northeastern Iran, and Old Persian, which belongs to the southwest, together constitute what is called Old Iranian."[f 1] Scholars traditionally classify Iranian languages as "old", "middle" and "new" according to their age, and as "eastern" or "western" according to geography, and within this framework Avestan is classified as Eastern Old Iranian. But the east–west distinction is of limited meaning for Avestan, as the linguistic developments that later distinguish Eastern from Western Iranian had not yet occurred. Avestan does not display some typical (South-)Western Iranian innovations already visible in Old Persian, and so in this sense, "eastern" only means "non-western".
The Avestan language is attested in roughly two forms, known as "Old Avestan" (or "Gathic Avestan") and "Younger Avestan". Younger Avestan did not evolve from Old Avestan; the two differ not only in time, but are also different dialects. Every Avestan text, regardless of whether originally composed in Old or Younger Avestan, underwent several transformations. Karl Hoffmann traced the following stages for Avestan as found in the extant texts. In roughly chronological order:
Many phonetic features cannot be ascribed with certainty to a particular stage since there may be more than one possibility. Every phonetic form that can be ascribed to the Sasanian archetype on the basis of critical assessment of the manuscript evidence must have gone through the stages mentioned above so that "Old Avestan" and "Young Avestan" really mean no more than "Old Avestan and Young Avestan of the Sasanian period."
The script used for writing Avestan developed during the 3rd or 4th century AD. By then the language had been extinct for many centuries, and remained in use only as a liturgical language of the Avesta canon. As is still the case today, the liturgies were memorized by the priesthood and recited by rote.
The script devised to render Avestan was natively known as Din dabireh "religion writing". It has 53 distinct characters and is written right-to-left. Among the 53 characters are about 30 letters that are – through the addition of various loops and flourishes – variations of the 13 graphemes of the cursive Pahlavi script (i.e. "Book" Pahlavi) that is known from the post-Sassanian texts of Zoroastrian tradition. These symbols, like those of all the Pahlavi scripts, are in turn based on Aramaic script symbols. Avestan also incorporates several letters from other writing systems, most notably the vowels, which are mostly derived from Greek minuscules. A few letters were free inventions, as were also the symbols used for punctuation. Also, the Avestan alphabet has one letter that has no corresponding sound in the Avestan language; the character for /l/ (a sound that Avestan does not have) was added to write Pazend texts.
The Avestan script is alphabetic, and the large number of letters suggests that its design was due to the need to render the orally recited texts with high phonetic precision. The correct enunciation of the liturgies was (and still is) considered necessary for the prayers to be effective.
The Zoroastrians of India, who represent one of the largest surviving Zoroastrian communities worldwide, also transcribe Avestan in Brahmi-based scripts. This is a relatively recent development first seen in the ca. 12th century texts of Neryosang Dhaval and other Parsi Sanskritist theologians of that era, which are roughly contemporary with the oldest surviving manuscripts in Avestan script. Today, Avestan is most commonly typeset in the Gujarati script (Gujarati being the traditional language of the Indian Zoroastrians). Some Avestan letters with no corresponding symbol are synthesized with additional diacritical marks, for example, the /z/ in zaraϑuštra is written with j with a dot below.
Avestan has retained voiced sibilants, and has fricative rather than aspirate series. There are various conventions for transliteration of Dīn Dabireh, the one adopted for this article being:
The glides y and w are often transcribed as ii and uu, imitating Dīn Dabireh orthography. The letter transcribed t̰ indicates an allophone of /t/ with no audible release at the end of a word and before certain obstruents.
According to Beekes, [ð] and [ɣ] are allophones of /θ/ and /x/ respectively (in Old Avestan).