The Byzantine Empire (or Byzantium) was the Greek-speaking Eastern Roman Empire of the Middle Ages, centered around its capital of Constantinople, and ruled by the Byzantine emperors in direct succession to their ancient Roman predecessors. It was called the Roman Empire and also Romania (Greek: Ῥωμανία, Rhōmanía) by its inhabitants and neighbours. As the distinction between "Roman Empire" and "Byzantine Empire" is purely 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 (alternatively "New Rome").

During its existence of more than a thousand years the Empire remained one of the most powerful economic, cultural, and military forces in Europe, despite setbacks and territorial losses, especially during the Roman–Persian and Byzantine–Arab Wars. The Empire recovered during the Macedonian dynasty, rising again to become a pre-eminent power in the Eastern Mediterranean by the late tenth century, rivaling the Fatimid Caliphate. After 1071, however, much of Asia Minor, the Empire's heartland, was lost to the Seljuk Turks. The Komnenian restoration regained some ground and briefly re-established dominance in the twelfth century, but following the death of Andronikos I Komnenos and the end of the Komnenos dynasty in the late twelfth century the Empire declined again. The Empire received a mortal blow in 1204 by the Fourth Crusade, when it was dissolved and divided into competing Byzantine Greek and Latin realms. Despite the eventual recovery of Constantinople and re-establishment of the Empire in 1261, under the Palaiologan emperors, successive civil wars in the fourteenth century further sapped the Empire's strength. Most of its remaining territory was lost in the Byzantine–Ottoman Wars, culminating in the Fall of Constantinople and its remaining territories to the Muslim Ottoman Turks in the fifteenth century.