Istanbul once known as Constantinople and Byzantium before that, is the most populous city in Turkey, and the country's economic, cultural, and historical center. Istanbul is a transcontinental city in Eurasia, straddling the Bosphorus strait 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 14 million residents. Istanbul is one of the World's most populous cities and ranks as world's 5th-largest city proper and the largest European city.
Founded on the Sarayburnu promontory around 660 BCE as Byzantium, the city now known as Istanbul developed to become one of the most significant cities in history. For nearly sixteen centuries following its reestablishment as Constantinople or New Rome in 330 CE, it served as an imperial capital for the Roman and 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 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 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, and infrastructure improvements have produced a complex transportation network.
Approximately 12.56 million 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 bid for the Summer Olympics five times in twenty years.
Present-day Istanbul is a flourishing city, with a constantly growing skyline which is one of the most prominent in all of Europe and Western Asia. New developments are constantly being implemented including new metro lines, residential buildings and underground transportation projects such as the Marmaray Tunnel which is the deepest underground tunnel in the world. Along with these developments, the city is increasingly becoming the centre of operations for many international organizations such as the Organization of the Black Sea Economic Cooperation, Turkic Council and D-8 Organization for Economic Cooperation.