Istanbul, formerly known as Constantinople, is the largest city in Turkey and the country's economic, cultural and historic center. The city straddles the Bosporus strait, and lies in both Europe and Asia, with a population of over 15 million residents, comprising 19% of the population of Turkey. Istanbul is the most populous city in Europe, and the world's fifteenth-largest city.
Founded as Byzantion by Megarian colonists in the 7th century BCE, and renamed by Constantine the Great first as New Rome (Nova Roma) during the official dedication of the city as the new Roman capital in 330 CE, which he soon afterwards changed to Constantinople (Constantinopolis), the city grew in size and influence, becoming a beacon of the Silk Road and one of the most important cities in history. It served as an imperial capital for almost sixteen centuries, during the Roman/Byzantine (330–1204), Latin (1204–1261), Byzantine (1261–1453), and Ottoman (1453–1922) empires. The city was instrumental in the advancement of Christianity during Roman and Byzantine times, hosting four (including Chalcedon (Kadıköy) on the Asian side) of the first seven ecumenical councils (all of which were in present-day Turkey), before its transformation to an Islamic stronghold following the Fall of Constantinople in 1453 CE, especially after becoming the seat of the Ottoman Caliphate in 1517. In 1923, after the Turkish War of Independence, Ankara replaced the city as the capital of the newly formed Republic of Turkey. In 1930, the city's name was officially changed to Istanbul, an appellation Greek speakers used since the eleventh century to colloquially refer to the city.Over 13.4 million foreign visitors came to Istanbul in 2018, eight years after it was named a European Capital of Culture, making it the world's eighth most visited city. Istanbul is home to several UNESCO World Heritage Sites, and hosts the headquarters of numerous Turkish companies, accounting for more than thirty percent of the country's economy.