
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, which it held for two more centuries. The borders of the empire fluctuated through several cycles of decline and recovery. In the reign of Heraclius ( r. 610–641), the Empire's military and administration were restructured and Greek was adopted for official use in place of Latin. Under Theodosius I ( r. 379–395), Christianity became the state religion and other religious practices were proscribed. Constantine I ( r. 324–337) reorganised the empire, made Constantinople the new capital and legalised Christianity. Several events from the 4th to 6th centuries mark the period of transition during which the Roman Empire's Greek East and Latin West diverged. Although the Roman state continued and its traditions were maintained, modern historians distinguish Byzantium from its earlier incarnation because it was centred on Constantinople, oriented towards Greek rather than Latin culture, and characterised by Eastern Orthodox Christianity. The Byzantines themselves continued to refer to their empire simply as the Roman Empire ( Medieval Greek: Βασιλεία Ῥωμαίων, romanized: Basileía Rhōmaíōn) or Romania ( Medieval Greek: Ῥωμανία), and to themselves as Romans ( Medieval Greek: Ῥωμαῖοι, romanized: Rhōmaîoi) – a term which Greeks continued to use for themselves into Ottoman times. "Byzantine Empire" is a term coined in 1557 by German historian Hieronymus Wolf, over a century after the fall of Constantinople and the end of the empire. During most of its existence, the empire was the most powerful economic, cultural, and military force in Europe. 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.

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.
