Greek for 'city of Constantine' — renamed in 330 CE when a Roman emperor refounded ancient Byzantium.
The historic name for the city now called Istanbul, founded by the Roman Emperor Constantine I in 330 CE as the new capital of the Roman Empire.
From Greek 'Kōnstantinoupolis' (Κωνσταντινούπολις, city of Constantine), composed of the personal name 'Kōnstantinos' (Constantine) + 'polis' (city). The city was founded on the site of the ancient Greek colony Byzantion (Byzantium) in 330 CE when Emperor Constantine I chose it as the new capital of the Roman Empire. The name follows the Greek pattern of 'person + polis' used for city names: just as many Greek cities bore names ending in '-polis,' Constantinople was literally 'Constantine's City.' The city was also known as 'the New Rome' and, in Greek, simply as 'hē Polis' (the City) — reflecting its preeminence.
The name 'Istanbul' may itself derive from Greek. One theory traces it to the Medieval Greek phrase 'eis tēn Polin' (εἰς τὴν Πόλιν, 'to the City' or 'in the City') — where 'Polin' is the accusative form of 'polis.' Greeks simply called Constantinople 'the City' because no other city rivaled it. Turkish speakers hearing 'eis tēn Polin' would have