Mosaic of the Virgin and Child with Emperor John Comnenus and Empress Irene (detail)

The Byzantine Empire (or Byzantium) was the Eastern Roman Empire during the periods of Late Antiquity and the Middle Ages, centred on the capital of Constantinople. Known simply as the "Roman Empire" (Greek: Βασιλεία Ῥωμαίων, Basileia Rhōmaiōn) or Romania (Ῥωμανία) to its inhabitants and neighbours, it was the direct continuation of the Ancient Roman State and maintained Roman state traditions. Byzantium is today distinguished from ancient Rome proper insofar as it was oriented towards Greek culture, characterised by Christianity rather than Roman paganism and was predominantly Greek-speaking rather than Latin-speaking.

As the distinction between Roman Empire and Byzantine Empire is largely a modern convention, it is not possible to assign a date of separation, but an important point is Emperor Constantine I's transfer in 324 of the capital from Nicomedia (in Anatolia) to Byzantium on the Bosphorus, which became Constantinople, "City of Constantine" (alternatively "New Rome"). The Roman Empire was finally divided in 395 AD after the death of Emperor Theodosius I (r. 379–395), thus this date is also very important if the Byzantine Empire (or Eastern Roman Empire) is looked upon as completely separated from the West. The transition to Byzantine history proper finally begins during the reign of Emperor Heraclius (r. 610–641), since Heraclius effectively established a new state after reforming the army and administration by introducing themes and by replacing the official language of the Empire from Latin to Greek.