English 'Istanbul' comes from Turkish İstanbul, which derives from the Medieval Greek phrase eis tēn Pólin — 'to the City' — because Constantinople was so important that Greeks just called it 'the City'.
The largest city in Turkey, straddling the Bosporus strait between Europe and Asia, formerly known as Constantinople and Byzantium.
English 'Istanbul' derives from Turkish 'İstanbul', which most scholars trace to Medieval Greek 'εἰς τὴν Πόλιν' (eis tēn Pólin), meaning 'to the City' or 'in the City'. Constantinople was so dominant in the eastern Mediterranean that Greeks simply called it 'the City' (ἡ Πόλις). The casual directional phrase — 'I'm going to the City' — was borrowed into Turkish as a proper name. This etymology, supported by most modern linguists, explains the initial 'i-' as a Turkish rendering
Istanbul is one of the few major cities whose name is a frozen phrase from another language. The Greeks called Constantinople simply 'the City' — and the Turkish name Istanbul preserves a Greek directional phrase meaning 'to the City', as if the name were an overheard snippet of Greek conversation turned into a place name.