Wikipedia
Fahlavīyāt (sing.: fahlavīya, Arabicized form of Persian pahlavī, in its original sense of Parthian), a designation given especially to the quatrains and by extension to the poetry in general composed in the old dialects of the Pahla/Fahla regions.
According to Ibn Nadeem who quotes Ebn al-Moqaffaʿ in his Al-Fihrist, Fahla consisted of five regions, namely Isfahan, Ray, Hamadan, Māh Nehāvand, and Azerbaijan. Khwarizmi describes it as a region consisting of Ray, Isfahan, Hamadan, Dinavar, Nehāvand, Mehrajān-qaḏaq, Māsabaḏān, and Qazvin. The use of fahla (< Mid. Pers. pahlaw) for designating Media goes back to late Arsacid times .
The specimens of fahlavī poems quoted in Persian texts are mostly attributed to the above-mentioned regions. Nevertheless, from the linguistic point of view the Fahla area may have extended to Gilan. Thus fahlaviyat include poems composed in the former Iranian dialects of western, central, and northern Persia.