Istanbul is a city.
Istanbul ( [ ˌɪstænˈbʊl ] , [ - ˈbuːl ] or [ - stɑːn - ] İstanbul [ isˈtɑnbuɫ ] ) , historically known as Constantinople and Byzantium , is the most populous city in Turkey and the country 's economic , cultural , and historic center . Istanbul is a transcontinental city in Eurasia , straddling the Bosphorus strait ( which separates Europe and Asia ) between the Sea of Marmara and the Black Sea . Istanbul is one of the world 's most populous cities and ranks as the world 's 7th-largest city proper and the largest European city .