In Latin, Gallia was also sometimes used as a general term for all Celtic peoples and their territories, such as all Brythons, including Germanic and Iberian provinces that also had a population with a Celtic culture. The plural, Galliarum in Latin, indicates that all of these are meant, not just Caesar's Gaul (several modern countries).
Gallia covered about half of the Gallic provinces of the early empire:
Viennensis was named after the city of Vienna (now Vienne), and almost entirely in present-day France, roughly south of the Loire. It was originally part of Caesar's newly conquered province of Transalpine Gaul, but a separate diocese from the start.
In the fifth century, Viennensis was replaced by a diocese of Septem Provinciae ('7 Provinces') with similar boundaries.
Hispania was the name of the whole Iberian Peninsula. It covered Hispania and the westernmost province of Roman Africa:
Originally there was a single diocese of Italia, but it was eventually split into a northern section and a southern section. The division of Italy into regions had already been established by Aurelian.
Suburbicaria indicates proximity to Rome, the Urbs (capital city). It included the islands, which were previously considered outside Italy.
Annonaria refers to a reliance on the area for the provisioning of Rome. It encompassed northern Italy and Raetia.
Africa included the central part of Roman North Africa:
The Prefecture of Illyricum was named after the former province of Illyricum. It originally included two dioceses, the Diocese of Pannonia and the Diocese of Moesia. Constantine I later split the Diocese of Moesia into two dioceses: the Diocese of Macedonia and the Diocese of Dacia.
Pannonia was one of the two dioceses in the eastern quarters of the Tetrarchy not belonging to the cultural Greek half of the empire (the other was Dacia); It was transferred to the western empire when Theodosius I fixed the final split of the two empires in 395.
The Dacians had lived in the Transylvania area, annexed to the Empire by Trajan. However, during the invasions of the third century Dacia was largely abandoned[why?]. Some inhabitants evacuated from the abandoned province settled on the south side of the Danube. They renamed their new homeland Dacia to diminish the impact that abandoning the original Dacia had on the Empire's prestige. The diocese was transferred to the western empire in 384 by Theodosius I, probably in partial compensation to the empress Justina for his recognition of the usurpation of Magnus Maximus in Britannia, Gaul and Hispania.
The Diocese of Macedonia was transferred to the western empire in 384 by Theodosius I, probably in partial compensation to the empress Justina for his recognition of the usurpation of Magnus Maximus in Britannia, Gaul and Hispania.
As the rich home territory of the eastern emperor, the Oriens ("East") prefecture would persist as the core of the Byzantine Empire long after the fall of Rome. Its praetorian prefect would be the last to survive, but his office was transformed into an essentially internal minister, stripped of its original military function.
Thrace was the easternmost corner of the Balkans (the only part outside the Illyricum prefecture) and the European hinterland of Constantinople.
Asia (or Asia Minor) in Antiquity stood for Anatolia. This diocese (the name means 'the Asian ones') centred on the earlier Roman province of Asia, and only covered the rich western part of the peninsula, mainly near the Aegean Sea.
It mainly contains parts of Asia minor near those coasts (as well as the mountainous centre), but also includes the north of very variable border with Rome's enemy Parthia/Persia.
The Eastern diocese shared its geographic name with the prefecture it belonged to, even after it lost its richest part, Egypt, becoming a separate diocese; but militarily crucial on the Persian (Sassanid) border and unruly desert tribes.
Further it contained the southeastern coast of Asia Minor and the close island of Cyprus
This diocese, comprising northeastern Africa—mainly Egypt, the rich granary and traditional personal domain of the emperors—was the only diocese that was not under a vicarius, but whose head retained the unique title of Praefectus Augustalis. It was created by a split of the Diocese of Oriens.
All but one, the civilian governors were of the modest rank of Praeses provinciae.
Apart from modern Egypt, Aegyptus also comprised the former province of Cyrenaica, being the east of modern Libya (an ancient name for the whole African continent as well). Cyrenaica was split into two provinces, each under a praeses: