Praetorian prefectures

The praetorian prefecture (Latin: praefectura praetorio, in Greek variously named ἐπαρχότης τῶν πραιτωρίων or ὑπαρχία τῶν πραιτωρίων) was the largest administrative division of the late Roman Empire, above the mid-level dioceses and the low-level provinces. Praetorian prefectures originated in the reign of Constantine I (r. 306-337), reaching their more or less final form in the last third of the 4th century and surviving until the 7th century, when the reforms of Heraclius diminished the prefecture's power, and the Muslim conquests forced the East Roman Empire to adopt the new theme system. Elements of the prefecture's administrative apparatus however are documented to have survived in the Byzantine Empire until the first half of the 9th century.

The office of the praetorian prefect had a long history dating back to the origins of the Roman Empire: initially, its two holders were the commanders of the Praetorian Guard, but gradually, they became the emperor's chief aides, and amassed considerable administrative and judicial responsibilities. The exact process of transformation to the chief civilian administrator of a specific territorial circumscription is still unclear. A common misconception, based on Zosimus, is that Constantine I established the praetorian prefectures as definite territorial administrations as early as 318, or in 324, after his victory over Licinius.

...
Wikipedia