As Constantinople, Istanbul was the capital of the Eastern or Byzantine Empire.
empire ( 395-1453) in SE Europe & SW Asia, formed by the division of the Roman Empire: cap. Constantinople
also Eastern Empire
An empire of the eastern Mediterranean region, dating from AD 395 when the Roman Empire was partitioned into eastern and western portions. Its extent varied greatly over the centuries, but its core remained the Balkan Peninsula and Asia Minor. The empire collapsed when its capital, Constantinople, fell to the Ottoman Turks in 1453.