Praetorian prefect

Praetorian prefect (Latin: praefectus praetorio, Greek: ἔπαρχος/ὕπαρχος τῶν πραιτωρίων) was the title of a high office in the Roman Empire. Originating as the commander of the Praetorian Guard, the office gradually acquired extensive legal and administrative functions, with its holders becoming the Emperor's chief aides. Under Constantine I, the office was much reduced in power and transformed into a purely civilian administrative post, while under his successors, territorially-defined praetorian prefectures emerged as the highest-level administrative division of the Empire. The prefects again functioned as the chief ministers of the state, with many laws addressed to them by name. In this role, praetorian prefects continued to be appointed until the reign of Heraclius in the 7th century AD, when wide-ranging reforms reduced its power and converted it to a mere overseer of provincial administration. The last traces of the prefecture disappeared in the Byzantine Empire by the 840s.

The term praefectus praetorio was often abbreviated in inscriptions as 'PR PR' or 'PPO'.

Under the empire the praetorians or imperial guards were commanded by one, two, or even three praefects (praefecti praetorio), who were chosen by the emperor from among the equites and held office at his pleasure. From the time of Alexander Severus the post was open to senators also, and if an equestrian was appointed he was at the same time raised to the senate. Down to the time of Constantine, who deprived the office of its military character, the prefecture of the guards was regularly held by tried soldiers, often by men who had fought their way up from the ranks. In course of time the command seems to have been enlarged so as to include all the troops in Italy except the corps commanded by the city praefect (cohortes urbanae).

The special position of the praetorians made them a power in their own right in the Roman state, and their prefect, the praefectus praetorio, soon became one of the more powerful men in this society. The emperors tried to flatter and control the praetorians, but they staged many coups d'état and contributed to a rapid rate of turnover in the imperial succession. The praetorians thus came to destabilize the Roman state, contrary to their purpose. The praetorian prefect became a major administrative figure in the later empire, when the post combined in one individual the duties of an imperial chief of staff with direct command over the guard also. Diocletian greatly reduced the power of these prefects as part of his sweeping reform of the empire's administrative and military structures.

...
Wikipedia