Origin and overview
As a written language, Old Persian is attested in royal Achaemenid inscriptions. It is an Iranian language and as such a member of the Indo-Iranian branch of the Indo-European language family. The oldest known text written in Old Persian is from the Behistun Inscriptions. Old Persian is one of the oldest Indo-European languages which are attested in original texts.
The oldest date of use of Old Persian as a spoken language is not precisely known. According to certain historical assumptions about the early history and origin of ancient Persians in southwestern Iran (where Achaemenids hailed from), Old Persian was originally spoken by a tribe called Parsuwash, who arrived in the Iranian Plateau early in the 1st millennium BCE and finally migrated down into the area of present-day Fārs province. Their language, Old Persian, became the official language of the Achaemenid kings. Assyrian records, which in fact appear to provide the earliest evidence for ancient Iranian (Persian and Median) presence on the Iranian Plateau, give a good chronology but only an approximate geographical indication of what seem to be ancient Persians. In these records of the 9th century BCE, Parsuwash (along with Matai, presumably Medians) are first mentioned in the area of Lake Urmia in the records of Shalmaneser III. The exact identity of the Parsuwash is not known for certain, but from a linguistic viewpoint the word matches Old Persian pārsa itself coming directly from the older word *pārćwa. Also, as Old Persian contains many words from another extinct Iranian language, Median, according to P. O. Skjærvø it is probable that Old Persian had already been spoken before formation of the Achaemenid Empire and was spoken during most of the first half of the first millennium BCE. Xenophon, a Greek general serving in some of the Persian expeditions, describes many aspects of Armenian village life and hospitality in around 401 BCE, which is when Old Persian was still spoken and extensively used. He relates that the Armenian people spoke a language that to his ear sounded like the language of the Persians.