Istanbul (UK:, or US: or ; Turkish: İstanbul [isˈtɑnbuɫ] ( listen)), 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. Its commercial and historical center lies on the European side and about a third of its population lives on the Asian side. The city is the administrative center of the Istanbul Metropolitan Municipality (coterminous with Istanbul Province), both hosting a population of around 15 million residents. 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. Istanbul is viewed as a bridgebetween the East and West.
Founded under the name of Byzantion (Βυζάντιον) on the Sarayburnu promontory around 660 BCE, the city grew in size and influence, having become one of the most important cities in history. After its reestablishment as Constantinople in 330 CE, it served as an imperial capital for almost 16 centuries, during the Roman/Byzantine (330–1204 and 1261–1453), the Latin (1204–1261), and the Ottoman (1453–1922) empires. It was instrumental in the advancement of Christianity during Roman and Byzantine times, before the Ottomans conquered the city in 1453 CE and transformed it into an Islamic stronghold and the seat of the Ottoman Caliphate.
Istanbul's strategic position on the historic Silk Road, rail networks to Europe and the Middle East, and the only sea route between the Black Sea and the Mediterranean have produced a cosmopolitan populace, although less so since the establishment of the Turkish Republic in 1923.
