"Constantinople" is an Anglicization of "Konstantinoupolis," which means "City of Constantine" in Greek, and was given that name in reference to the Roman emperor Constantine I when he made it the capital of the Roman Empire on May 11, 330 A.D. Constantine actually named it "Nova Roma", but that name never really got into common use. Rome retained its political and commercial privileges.
Constantinople and the Byzantine Empire finally fell to the Ottoman Empire on May 29, 1453. See the Fall of Constantinople. When the Republic of Turkey was founded in 1923, the capital moved from Istanbul to Ankara. Ottoman people were calling their capital city by various names, including Constantinople. Istanbul became the official name as late as 1930.