The Byzantine Empire was the Eastern Roman Empire, centered on the capital of Constantinople. Known simply as the Roman Empire to its inhabitants and neighbors, the Empire was the direct continuation of the Ancient Roman State. It is today distinguished from ancient Rome proper insofar as the Empire was oriented towards Greek culture, characterized by Christianity rather than Roman paganism, and predominantly Greek-speaking rather than Latin-speaking.
As the distinction between Roman Empire and Byzantine Empire is largely a modern convention, it is not possible to assign a date of separation, but an important point is Emperor Constantine I's transfer in 324 of the capital from Nicomedia to Byzantium, which became Constantinople, "City of Constantine". The Roman Empire was finally divided in 395 AD after the death of Emperor Theodosius I (r. 379–395), thus this date is also very important if the Byzantine Empire (or Eastern Roman Empire) is looked upon as completely separated from the West.
The Byzantine Empire existed for more than a thousand years, from the 4th century to 1453. During most of its existence, it remained one of the most powerful economic, cultural, and military forces in Europe, despite setbacks and territorial losses, especially during the Roman-Persian and Byzantine-Arab Wars. The Empire recovered during the Macedonian dynasty, rising again to become a preeminent power in the Eastern Mediterranean by the late 10th century, rivaling the Fatimid Caliphate.
After 1071, however, much of Asia Minor, the Empire's heartland, was lost to the Seljuk Turks. The Komnenian restoration regained some ground and briefly reestablished dominance in the 12th century, but following the death of Andronikos I Komnenos and the end of the Komnenos dynasty in the late 12th century the Empire declined again. The Empire received a mortal blow in 1204 from the Fourth Crusade, when it was dissolved and divided into competing Byzantine Greek and Latin realms.
Despite the eventual recovery of Constantinople and re-establishment of the Empire in 1261, under the Palaiologan emperors, Byzantium remained only one of many rival states in the area for the final 200 years of its existence. However, this period was the most culturally productive time in the Empire.
Successive civil wars in the 14th century further sapped the Empire's strength, and most of its remaining territories were lost in the Byzantine-Ottoman Wars, which culminated in the Fall of Constantinople and the conquest of remaining territories by the Ottoman Empire in the 15th century.