The Byzantine Empire was the predominantly Greek-speaking continuation of the Roman Empire during Late Antiquity and the Middle Ages. Its capital city was Constantinople (modern-day Istanbul), originally known as Byzantium. Initially the eastern half of the Roman Empire (often called the Eastern Roman Empire in this context), it survived the 5th century fragmentation and collapse of the Western Roman Empire and continued to thrive, existing for an additional thousand years until it fell to the Ottoman Turks in 1453. During most of its existence, the empire was the most powerful economic, cultural, and military force in Europe. Both "Byzantine Empire" and "Eastern Roman Empire" are historiographical terms applied in later centuries; its citizens continued to refer to their empire as the Roman Empire (Ancient Greek: Βασιλεία Ῥωμαίων, tr. Basileia Rhōmaiōn; Latin: Imperium Romanum), and Romania (Ῥωμανία).

Several events from the 4th to 6th centuries mark the transitional period during which the Roman Empire's east and west divided. In 285, the emperor Diocletian (r. 284–305) partitioned the Roman Empire's administration into eastern and western halves. Between 324 and 330, Constantine I (r. 306–337) transferred the main capital from Rome to Byzantium, later known as Constantinople ("City of Constantine") and Nova Roma ("New Rome"). Under Theodosius I (r. 379–395), Christianity became the Empire's official state religion and others such as Roman polytheism were proscribed. And finally, under the reign of Heraclius (r. 610–641), the Empire's military and administration were restructured and adopted Greek for official use instead of Latin. In summation, Byzantium is distinguished from ancient Rome proper insofar as it was oriented towards Greek rather than Latin culture, and characterised by Orthodox Christianity rather than Roman polytheism.