The Byzantine Empire (395-1453), also known as the Eastern Roman Empire, was an imperial state that believed in Orthodox Christianity. The core area is located in the Balkans in southeastern Europe, and its territory also included western Asia and northern Africa, and at its peak it also included Italy, Syria, Palestine, Egypt, the Caucasus, and the Mediterranean coast of North Africa. It was the oldest monarchy in ancient and medieval Europe.

In 395 AD, the Roman Empire was divided into two parts, East and West, of which the Eastern Roman Empire lasted for nearly a millennium, during which time it was generally referred to simply as the "Roman Empire" (Latin: Imperium Romanum; Greek: Βασιλε?αΡωμα?ων). In the 17th century, historians in Western Europe introduced the term "Byzantine Empire" (Greek: Βυζαντιναυτοκρατορα, English: The Byzantine Empire) in order to distinguish the ancient Roman Empire from the medieval Roman Empire. The culture and religion of the Eastern Roman Empire had a great influence on today's Eastern European countries, and its preserved ancient Greek and Roman historical materials, writings and rational philosophical ideas also provided the most direct impetus for medieval Europe to break through the shackles of Catholic theocracy, triggering the Renaissance movement and profoundly influencing human history.

The Byzantine Empire had 12 dynasties and 93 emperors. The capital of the empire was New Rome (Latin: NovaRoma, Constantinople). In 1204, the imperial capital of Constantinople was captured and slaughtered by the Fourth Crusade until it was regained in 1261. On May 29, 1453, the Ottoman Sultan Mehmed II led an army into Constantinople (now renamed Istanbul), and the Eastern Roman Empire officially ended.

The Byzantine Empire or Eastern Roman Empire was the Eastern Roman Empire formed by the division of the Roman Empire in 395 AD.

After Heraclius' reforms, Byzantium's culture and language were mostly Greek. Militarily very different from ancient Rome, after Rome split into Eastern and Western Rome, Western Rome inherited the fighting style of the infantry phalanx of ancient Rome, while Eastern Rome began to try to replace infantry with heavy cavalry as the main force of battle. Beginning in the 17th century, Western historians introduced the term Byzantine Empire in order to distinguish the ancient Roman Empire from the medieval Roman Empire. The name comes from the ancient Greek colony of Byzantium, the predecessor of its capital, Constantinople (Istanbul).