The importance of Constantinople in the Ottoman world was also reflected by its Ottoman nickname Der Saadet meaning the 'Gate to Prosperity' in Ottoman Turkish. This reflected its status as the only major city in the vicinity. The name İstanbul ( Turkish pronunciation: ( listen), colloquially Turkish pronunciation: ) is commonly held to derive from the Medieval Greek phrase 'εἰς τὴν Πόλιν' (pronounced Greek pronunciation: ), which means 'to the city' and is how Constantinople was referred to by the local Greeks. Ḳosṭanṭīnīye ( Ottoman Turkish: قسطنطينيه) and İstanbul were the names used alternatively by the Ottomans during their rule. Constantinople remained the most common name for the city in the West until the 1930s, when Turkish authorities began to press for the use of 'Istanbul' in foreign languages. Constantinople comes from the Latin name Constantinus, after Constantine the Great, the Roman emperor who refounded the city in 324 CE. Modern excavations have raised the possibility that the name Byzantium might reflect the sites of native Thracian settlements that preceded the fully fledged town. Megarian colonists claimed a direct line back to the founders of the city, Byzas, the son of the god Poseidon and the nymph Ceroëssa. The first known name of the city is Byzantium ( Greek: Βυζάντιον, Byzántion), the name given to it at its foundation by Megarian colonists around 657 BCE. 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. 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. In 1930, the city's name was officially changed to Istanbul, the Turkish rendering of the appellation Greek speakers used since the eleventh century to colloquially refer to the city. In 1923, after the Turkish War of Independence, Ankara replaced the city as the capital of the newly formed Republic of Turkey.

The city played a key role in the advancement of Christianity during Roman/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. The city served as an imperial capital for almost 1600 years: during the Roman/ Byzantine (330–1204), Latin (1204–1261), late Byzantine (1261–1453), and Ottoman (1453–1922) empires. The city grew in size and influence, eventually becoming a beacon of the Silk Road and one of the most important cities in history. In 330 CE, the Roman emperor Constantine the Great made it his imperial capital, renaming it first as New Rome ( Nova Roma) and then as Constantinople ( Constantinopolis) after himself. The city was founded as Byzantium ( Byzantion) in the 7th century BCE by Greek settlers from Megara. Istanbul is the most populous European city, and the world's 15th-largest city. The city straddles the Bosporus strait, lying in both Europe and Asia, and has a population of over 15 million residents, comprising 19% of the population of Turkey. Istanbul ( / ˌ ɪ s t æ n ˈ b ʊ l/ IST-an- BUUL, US also / ˈ ɪ s t æ n b ʊ l/ IST-an-buul Turkish: İstanbul ( listen)) is the largest city in Turkey, serving as the country's economic, cultural and historic hub.

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