The Byzantine Empire is the name given to the eastern part of the Roman Empire, which survived until well into the Middle Ages. First split into an Eastern and Western divisions by Constantine in the 300s A.D., the Eastern Empire went on to outlive it's Western half by 1000 years. The empire was multi-ethnic and predominantly Greek-speaking. The empire's heritage consisted of the Eastern Orthodox Church, Greek philosophy, and Roman law.
The empire's history was extremely violent as it was often at war with at least one foreign power. As a result, its borders fluctuated greatly over time, but it was always centered in Constantinople. While taking on offensive position to "reunite" with the west under Justinian in the 500's, for the most part the positiion of the Empire was defensive in nature, having little desire to increase its borders. For years the cultured Byzantine Empire was the protector of Dark Age Europe, repelling Islamic armies that would stretch to control lands from Morocco to India. An important element in Byzantine survival was Greek Fire, a substance that could burn even in water and was a terror to enemy fleets. The contents were such a carefully guarded secret that to this day they are unknown.
In 1453, the capital of the Byzantine Empire fell to the Ottoman Turks after a long cannon bombardament at a time when the Empire had been reduced to a mere shell of its former self. The conquerors renamed the city Istanbul.