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):

Byzantium
    n 1: an ancient city on the Bosporus founded by the Greeks; site
         of modern Istanbul; in 330 Constantine I rebuilt the city
         and called it Constantinople and made it his capital
    2: a continuation of the Roman Empire in the Middle East after
       its division in 395 [syn: Byzantine Empire, Byzantium,
       Eastern Roman Empire]