The Byzantine Empire or Eastern Roman Empire, was the Roman Empire during the Middle Ages, centered on the capital of Constantinople, and ruled by Emperors. It was called the Roman Empire, and also as Romania (Greek: Ῥωμανία, Rhōmanía), by its inhabitants and its 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 the 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").

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 the pre-eminent power in the Eastern Mediterranean by the late 10th century. 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 12th century, but declined again under their successors. 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 14th 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 15th century.