Middle Persian
Middle Persian or Pahlavi is the Iranian language spoken during Sassanian times. It descended from Old Persian.
Middle Persian was usually written in the Pahlavi script. The language was also written down in the Manichean script by Persian-speaking Manichees.
Transition to Modern Persian
The modern-day descendent of Middle Persian is Modern Persian. The latter is distinguished from the former by, among other things, being written in a different script, the Perso-Arabic script, and by a large number of Arabic loanwords. Pahlavi Middle Persian is the language of quite a large body of Zoroastrian literature which details the traditions and prescriptions of the Zoroastrian religion which was the state religion of Sassanid Iran (224 to ca. 650) before Iran was defeated by the Arab armies that introduced Islam.
See also: Pahlavi Literature