From the perspective of classical Greece, Thracia included the territory north of Thessaly, with no definite boundaries, sometimes to the inclusion of Macedonia and Scythia minor. Later, Thracia proper was understood to include the territory bordered by the Danube on the north, by the Black Sea on the east, by Macedonia in the south and by Illyria to the west, roughly equivalent with the territory of the Thracian kingdom as it stood during the 5th to 1st centuries BC.
With the annexation of the Thracian kingdom by the Roman Empire, by order of emperor Claudius, in AD 46, Thracia (formally provincia Thracia "Thracian province", ἐπαρχία Θρᾳκῶν "eparchy of the Thracians") was established as a Roman province. After the administrative reforms of the 3rd century, Thracia was reduced to the territory of the six small provinces of the
Diocese of Thrace. Later still, the medieval Byzantine theme of Thracia contained only what today is Eastern Thrace.