The Collaborative International Dictionary of English v.0.48:

Istanbul \Istanbul\ prop. n.
   A city in European Turkey, built on the site of ancient
   Byzantium. It is the former capital of the Turkish Empire,
   known as Constantinople before being captured by the Turks.

   Syn: Stambul, Stamboul, Constantinople.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Byzantium \Byzantium\ prop. n.
   An ancient city on the Bosphorus founded by the Greeks. It
   was later renamed Constaninople in honor of the emperor
   Constantine, and renamed Istanbul by the Turks, which name
   it still retains.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

Istanbul
    n 1: the largest city and former capital of Turkey; rebuilt on
         the site of ancient Byzantium by Constantine I in the
         fourth century; renamed Constantinople by Constantine who
         made it the capital of the Byzantine Empire; now the seat
         of the Eastern Orthodox Church [syn: Istanbul, Stambul,
         Stamboul, Constantinople]