Istanbul (is"tănbOOl', istan'bOOl) [key], city (1990 pop. 6,748,435), capital of Istanbul prov., NW Turkey, on both sides of the Bosporus at its entrance into the Sea of Marmara. Its name was officially changed from Constantinople to Istanbul in 1930; before A.D. 330 it was known as Byzantium. (For the history of the city, see Byzantium and Constantinople.)