Wikipedia
Middle Persian is the Middle Iranian language or ethnolect of southwestern Iran that during the Sasanian Empire (224–654) became a prestige dialect and so came to be spoken in other regions of the empire as well. Middle Persian is classified as a Western Iranian language. It descends from Old Persian and is the linguistic ancestor of Modern Persian.
Middle Persian consisted of several dialects and variants. One of these variants was called Pahlavīk (Pahlavi) which stands for Parthian, and refers to the Middle Persian that was the language of the Parthian Empire. Another variant of Middle Persian, known locally as Pārsik, was the official language of the Sasanian Empire. Most scholars refer to the latter variant when using the term "Middle Persian".
The native name for Middle Persian was Pārsīk (later Pārsīg) translating to "language of Pārs". It consists of Pārs (local name of the Persis province) + adjective suffix -īk ("having to do with"; from Proto-Indo-European -(i)ko and related to Greek –ikos, French –ique, Slavic –isku; e.g. Āsōrik "Assyrian", etc.). The word is consequently the origin of the native name for the Modern Persian language—Parsi or Fārsī.
Traces of Middle Persian, or Parsik, are found in remnants of Sasanian inscriptions and Egyptian papyri, coins and seals, fragments of Manichaean writings, and treatises and Zoroastrian books from the Sasanian era, as well as in the post-Sasanian Zoroastrian variant of the language sometimes known as Pahlavi, which originally referred to the Pahlavi scripts, and that was also the preferred writing system for several other Middle Iranian languages. Aside from the Aramaic alphabet-derived Pahlavi script, Zoroastrian Middle Persian was occasionally also written in Pazend, a system derived from the Avestan alphabet that, unlike Pahlavi, indicated vowels and did not employ logograms. Manichaean Middle Persian texts were written in the Manichaean alphabet, which also derives from Aramaic but in an Eastern Iranian form via the Sogdian alphabet.
The ISO 639 language code for Middle Persian is pal, which reflects the post-Sasanian era use of the term Pahlavi to refer to the language and not only the script.