The Byzantine Empire or Byzantium is the historiographical term conventionally used since the 19th century to describe the Greek-speaking Roman Empire of the Middle Ages, centered on its capital of Constantinople. During much of its history it was known to many of its Western contemporaries as the Empire of the Greeks because of the dominance of Greek language, culture and population.To its inhabitants, the Empire was simply the Roman Empire (Βασιλεία Ῥωμαίων) or Romania (Ῥωμανία) and its emperors continued the unbroken succession of Roman emperors. In the Islamic world it was known primarily as روم‎ (Rûm, land of the "Romans").

By many historians the Byzantine Empire is called Eastern Roman Empire until the reign of Heraclius. This discrimantion takes place because until then, the civilians of the empire continued to act and to think as the civilians of the Roman Empire were doing. But in these years many things happened and had many consequences which had as a result the creation of a new mentality much different from that of the Roman people.

There is no consensus on exactly when the Byzantine period of Roman history began. Many consider Emperor Constantine I (reigned AD 306–337) to be the first "Byzantine Emperor". It was he who moved the imperial capital in 324 AD from Nicomedia to Byzantium, refounded as Constantinople, or Nova Roma ("New Rome"). The city of Rome itself had not served as the capital since the reign of Diocletian (284-305).

Some date the beginnings of the Empire to the reign of Theodosius I (379–395) and Christianity's official supplanting of the pagan Roman religion, or following his death in 395, when the political division between East and West became permanent. Others place it even later in 476, when Romulus Augustulus, traditionally considered the last western Emperor, was deposed, thus leaving sole imperial authority with the emperor in the Greek East. Others point to the reorganization of the empire in the time of Heraclius (ca. 620), when Latin titles and usages were officially replaced with Greek versions.

In any case, the changeover was gradual and by 330, when Constantine inaugurated his new capital, the process of hellenization and increasing Christianization was already under way. The Empire is considered to have ended after the fall of Constantinople to the Ottoman Turks in 1453, although Greek monarchies continued to rule over parts of the fallen Empire's territories for several more years, until the fall of Mystras in 1460, Trebizond in 1461, and Monemvasia in 1471.