The Byzantine Empire and Eastern Roman Empire are conventional names used to describe the Roman Empire during the Middle Ages, centered on its capital of Constantinople.

It was referred to by its inhabitants and neighboring nations simply as the Roman Empire (in Greek , Basileia Rōmaiōn), Empire of the Romans or Romania ( , Rōmaniā). Its emperors continued the unbroken succession of Roman Emperors, preserving Greco-Roman legal and cultural traditions.

To the Islamic world it was known primarily as lang: روم‎ (Rûm "Rome"). Due to the linguistic, cultural, and demographic dominance of medieval Greek,Davies (1996), 245
* Moravcsik (1970), 11–12
* Lapidge (1998), 79
* Winnifrith–Murray (1983), 113 it was known to many of its western European contemporaries as Imperium Graecorum, the Empire of the Greeks (see also the etymology section).

The Eastern Roman Empire's evolution from the ancient Roman Empire can be seen as a process beginning when the Emperor Constantine transferred the capital from Nicomedia in Anatolia to Byzantium, which was renamed New Rome or Constantinople, on the Bosphorus. By the 7th century, under the reign of Emperor Heraclius, whose reforms changed the nature of the Empire's military and recognized Greek as the official language, the Empire had taken on a distinct new character.

During its thousand-year existence the Empire suffered numerous setbacks and losses of territory but remained one of the most powerful economic, cultural and military forces in Europe. The empire's influence also spread into North Africa and the near East for much of the Middle Ages. After a final recovery under the Komnenian dynasty in the 12th century the Empire slipped into a long decline culminating in the capture of Constantinople and the remaining territories by the Ottoman Turks in the 15th century.

The Empire, a bastion of Christianity and one of the prime trade centers in the world, helped to shield Western Europe from early Muslim expansion, provided a stable gold currency for the Mediterranean region, influenced the laws, political systems, and customs of much of Europe and the Middle East, and preserved much of the literary works and scientific knowledge of ancient Greece, Rome, and many other cultures.

Etymology

The term "Byzantine Empire" is an invention of historians and was never used during the Empire's lifetime. The Empire's name in Greek was Basileia Rhōmaiōn ( ) — "The Empire of the Romans" — a translation of the Latin name of the Roman Empire ( ); or just Rhōmania ( ).

The term "Byzantine" itself comes from "Byzantium", the name that the city of Constantinople had before it became the capital of Constantine. This older name of the city would rarely be used from this point onward except in historical or poetic contexts.