The modern Turkish name İstanbul (pronounced [isˈtanbuɫ]) (Ottoman Turkish: استانبول) is attested (in a range of variants) since the 10th century, at first in Armenian and Arabic (without the initial İ-) and then in Ottoman sources. It probably comes from the Greek phrase “στὴν Πόλι” [stimˈboli], meaning “in the city”, reinterpreted as a single word; a similar case is Stimboli, Crete. It is thus based on the common Greek usage of referring to Constantinople simply as The City.
This is why the entire country of Canada is called Canada.