The Byzantine Empire, also referred to as the Eastern Roman Empire or Byzantium, is the common name given to the surviving Roman Empire in its eastern provinces during Late Antiquity and the Middle Ages, when its capital city was Constantinople (modern-day Istanbul, and formerly Byzantium). It survived the fragmentation and fall of the Western Roman Empire in the 5th century AD and continued to exist 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. "Byzantine Empire" is a term created after the end of the realm; its citizens continued to refer to their empire simply as the Roman Empire (Greek: Βασιλεία Ῥωμαίων, tr. Basileia Rhōmaiōn; Latin: Imperium Romanum), or Romania (Ῥωμανία), and to themselves as "Romans". In medieval Western Europe it was sometimes labelled the Imperium Graecorum (Empire of the Greeks) since the Holy Roman Empire, created in 800 AD by Charlemagne and Pope Leo III, was believed to represent the legitimate Roman Empire.
Several signal events from the 4th to 6th centuries mark the period of transition during which the Roman Empire's Greek East and Latin West diverged. Constantine I (r. 324–337) reorganised the empire, made Constantinople the new capital, and legalised Christianity. Under Theodosius I (r. 379–395), Christianity became the Empire's official state religion and other religious practices were proscribed. Finally, under the reign of Heraclius (r. 610–641), the Empire's military and administration were restructured and adopted Greek for official use in place of Latin. Thus, although the Roman state continued and its traditions were maintained, modern historians distinguish Byzantium from ancient Rome insofar as it was centred on Constantinople, oriented towards Greek rather than Latin culture, and characterised by Eastern Orthodox Christianity.
The borders of the empire evolved significantly over its existence, as it went through several cycles of decline and recovery. During the reign of Justinian I (r. 527–565), the empire reached its greatest extent after reconquering much of the historically Roman western Mediterranean coast, including North Africa, Italy, and Rome itself, which it held for two more centuries. The Byzantine–Sasanian War of 602–628 exhausted the empire's resources and contributed to major territorial losses during the Early Muslim conquests of the 7th century, when it lost its richest provinces, Egypt and Syria, to the Arab caliphate. During the Macedonian dynasty (10th–11th centuries), the empire expanded again and experienced the two-century long Macedonian Renaissance, which came to an end with the loss of much of Asia Minor to the Seljuk Turks after the Battle of Manzikert in 1071. This battle opened the way for the Turks to settle in Anatolia.
The empire recovered during the Komnenian restoration, and by the 12th century Constantinople was the largest and wealthiest European city. However, it was delivered a mortal blow during the Fourth Crusade, when Constantinople was sacked in 1204 and the territories that the empire formerly governed were divided into competing Byzantine Greek and Latin realms. Despite the eventual recovery of Constantinople in 1261, the Byzantine Empire remained only one of several small rival states in the area for the final two centuries of its existence. Its remaining territories were progressively annexed by the Ottomans over the 14th and 15th century. The Fall of Constantinople to the Ottoman Empire in 1453 finally ended the Byzantine Empire. The last of the imperial Byzantine successor states, the Empire of Trebizond, would be conquered by the Ottomans eight years later in the 1461 Siege of Trebizond.