Claim: Istanbul was once named Byzantion.

Istanbul -LRB- -LSB- ˌɪstænˈbʊl -RSB- , -LSB- - ˈbuːl -RSB- or -LSB- - stɑːn - -RSB- İstanbul -LSB- isˈtɑnbuɫ -RSB- -RRB- , 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 -LRB- which separates Europe and Asia -RRB- 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 -LRB- conterminous with Istanbul Province -RRB- , both hosting a population of around 14.7 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 .   Founded under the name of Byzantion on the Sarayburnu promontory around 660 BCE , the city developed to become one of the most significant in history . After its reestablishment as Constantinople in 330 CE , it served as an imperial capital for almost 16 centuries , during the Roman and Byzantine -LRB- 330 -- 1204 and 1261 -- 1453 -RRB- , the Latin -LRB- 1204 -- 1261 -RRB- , and the Ottoman -LRB- 1453 -- 1922 -RRB- empires . It was instrumental in the advancement of Christianity during Roman and Byzantine times , before the Ottomans conquered the city in 1453 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 . Overlooked for the new capital Ankara during the interwar period , the city has since regained much of its prominence . The population of the city has increased tenfold since the 1950s , as migrants from across Anatolia have moved in and city limits have expanded to accommodate them . Arts , music , film , and cultural festivals were established at the end of the 20th century and continue to be hosted by the city today . Infrastructure improvements have produced a complex transportation network .   Approximately foreign visitors arrived in Istanbul in 2015 , five years after it was named a European Capital of Culture , making the city the world 's fifth most popular tourist destination . The city 's biggest attraction is its historic center , partially listed as a UNESCO World Heritage Site , and its cultural and entertainment hub can be found across the city 's natural harbor , the Golden Horn , in the Beyoğlu district . Considered a global city , Istanbul has one of the fastest-growing metropolitan economies in the world . It hosts the headquarters of many Turkish companies and media outlets and accounts for more than a quarter of the country 's gross domestic product . Hoping to capitalize on its revitalization and rapid expansion , Istanbul has bid for the Summer Olympics five times in twenty years .