Istanbul, formerly known as Byzantium and Constantinople, is a transcontinental city in Eurasia, straddling the Bosporus strait (which separates Europe and Asia) between the Sea of Marmara and the Black Sea. Its commercial and historical center lies on the European side and about a third of its population lives in suburbs on the Asian side of the Bosporus. With a total population of around 15 million residents in its metropolitan area, Istanbul is one of the world's largest cities, ranking as the world's fifth-largest city proper and the largest city in Europe.
Founded under the name of Byzantion (Βυζάντιον), the city grew in size and influence, becoming one of the most important cities in history. Under the name Constantinople it was the Ottoman capital until 1923. The capital was then moved to Ankara and the city was renamed Istanbul.
Thank you, Wikipedia.