What is "arbayistan"

Arbāyistān ( Syriac: Bēṯ ʿArbāyē, Armenian: Arvastan) was a Sassanian province in Late Antiquity, that bordered the Roman Empire, and later the Byzantine Empire, and was a constant area of contention between the Romans and the Sassanians.

The province reached across Upper Mesopotamia toward the Khabur and north to the lower districts of Armenia; it bordered Adiabene in the east, Armenia in the north and Assuristan in the south.