praetorian prefect (praefectus praetorio)
Commander of the emperor's bodyguard under the principate. During the regne of Constantine I the praetorian prefect becomes a dignitary responsible for the administrative unit called the prefecture, which was subdivided into dioceses. In 400 A.D. there were four such praetorian prefectures, of Oriens, of Illyricum, of Illyricum, Italia and Africa and of Gallia. The praetorian prefects were second only to the emperor. The praetorian prefect of Oriens was the mightiest among prefects. His office is for the last time mentioned in 680.