The etymology of Byzantium is unknown. It has been suggested that the name is of Thraco-Illyrian origin. It may be derived from the Thracian or Illyrian personal name Byzas. Ancient Greek legend refers to King Byzas, the leader of the Megarian colonists and founder of the city.
The form Byzantium is a latinisation of the original name.
Much later, the name Byzantium became common in the West to refer to the Eastern Roman Empire. Its capital Constantinople stood on the site of ancient Byzantium. The name "Byzantine Empire" was introduced by the historian Hieronymus Wolf only in 1555, a century after the empire had ceased to exist. While the empire existed, the term Byzantium referred to only the city, rather than the empire.
The name Lygos for the city, which likely corresponds to an earlier Thracian settlement, is mentioned by Pliny the Elder in his Natural History.