As Constantinople, Istanbul was the capital of the Eastern or Byzantine Empire.
Byzantine Empire definition by Webster's New World
Byzantine Empire definition by American Heritage Dictionary
also Eastern Empire
Byzantine Empire - Cultural Definition
An empire, centered at Constantinople, that began as the eastern portion of the Roman Empire; it included parts of Europe and western Asia. As the western Roman Empire declined, the Byzantine Empire grew in importance, and it remained an important power in Europe until the eleventh century. The Byzantine Empire was conquered by Turkish forces in the fifteenth century.
The Byzantine emperor was an absolute ruler (see absolute monarchy), and the laws and customs associated with his empire were strict and complex. His rule was supported by the Christian Church in the region, which later became the independent Eastern Orthodox Church.