The Byzantine Empire, also referred to as the Eastern Roman Empire, or Byzantium, was the continuation of the Roman Empire in its eastern provinces during Late Antiquity and the Middle Ages, when its capital city was Constantinople. 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 Empire 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, or Romania, and to themselves as Romans.
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 reorganised the empire, made Constantinople the new capital and legalised Christianity. Under Theodosius I, Christianity became the state religion and other religious practices were proscribed.