Istanbul, originally Constantinople and Byzantium, is the Ottoman capital in the Civilization games and a city in modern day Turkey. Constantinople is the capital of the Byzantine empire in Civilization games. Byzantium is a possible city name for the Roman civilization in the original Civilization game.
The city was originally a Greek city named Byzantium. The Roman Emperor, Constantine, chose the city to be the capital of the Eastern portion of the Roman Empire, which later became the Byzantine Empire. The city was renamed in his honor. The city eventually was conquered by the Ottoman empire in 1453. The city was officially renamed "Istanbul" (derived from the Greek words "eis tin polin," meaning "to the city") after the Turkish War of Independence in 1923. The Turkish capital was moved to the city Ankara that same year.
Confusingly, the city was never named Istanbul while the entity known as the Ottoman empire controlled the city. Moreover, the Byzantine empire was a name given to the Eastern Roman Empire after it fell. Byzantine citizens called themselves Romans, and their own territory the Roman Empire.