The Eastern Roman Empire or the Eastern Empire was the name given to the eastern part of the Roman Empire after its division in the 3rd century AD.
Its capital city was Constantinople (or New Rome). It represented an administrative division of the Roman Empire, but after the fall of the western part it survived until the fall of Constantinople in 1453. The Eastern Empire was also called the Byzantine Empire. The term Byzantine Empire dates from the 17th century. 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 and its emperors continued the unbroken succession of Roman emperors.
Byzantine Empire
The division of the Empire began with the Tetrarchy (quadrumvirate) in the late 3rd century AD with Diocletian, as an institution that was aiming to control efficiently the vast Roman empire.
Some date the beginnings of the Empire to the reign of Theodosius I (379A 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 yet 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 generally considered to have ended after the fall of Constantinople to the Ottoman Turks in 1453, although some scholars instead use the Sack of Constantinople in 1204, or the fall of the Greek successor kingdoms, Mystras in 1460, Trebizond in 1461, and Monemvasia in 1471.