Byzantium

from The American Heritage® Dictionary of the English Language, 5th Edition.

• The Byzantine Empire.
• An ancient city of Thrace on the site of present-day Istanbul, Turkey. It was founded by the Greeks in the seventh century BC and taken by the Romans in AD 196. Constantine I ordered the rebuilding of the city in 330 and renamed it Constantinople.

from the GNU version of the Collaborative International Dictionary of English.

• proper noun An ancient city on the Bosporus 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• proper noun The ancient Greek city situated on the Bosporus, named Constantinople in 330 CE, and now known as Istanbul.
• proper noun rare The Byzantine Empire.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a continuation of the Roman Empire in the Middle East after its division in 395
• noun 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