Praefectus

A praefectus was a regional governor, in whom the Caesar placed his trust and authority for the administration of a part of the Roman Empire. Ian Chesterton once met the praefectus of Byzantium, Thalius Maximus. Thalius lived in Villa Praefectus, an opulent residence which was full of political intrigue, much like royal courts would be in later centuries in Europe. After spending some time observing Thalius in the villa, Ian came to better understand the cultural debt Nazi Germany owed to the Roman Empire. (PDA: Byzantium!)

In the real world, a praefectus was not always a governor, but a more general term for an executive official, most often junior to a regional governor. Thalius, however, is explicitly called "the governor protector of the coloniae civiura Rontanorum Byzantium."