Nicomedia was a city in Anatolia, near Constantinople and Nicea, that was the interim capital for Constantine the Great while the city of Byzantium was re-built and became Constantinople. During the early days after Roman recognition of Christianity Nicomedia was a see serving the imperial household. In modern days the city continues to exist as Izmit in the Turkish Republic.
The site of Nicomedia (Greek: Νικομήδεια) has been known by other names. The oldest city was founded on the site in the eighth century before Christ. Initially known as Astacus or Olbia, the city was rebuilt by Nicomedes I of Bithnia in the third century before Christ and named Nicomedia. Nicomedia became one of the important cities in northwestern Asia Minor. With the introduction of the tetrarchy system of government for the Roman empire, Diocletian made the city the capital of the eastern part of the empire in 286.
After defeating Licinius in 324, Constantine used Nicomedia as his interim capital. He died in the royal villa in 337 which was near Nicomedia. Nicomedia, with nearby Nicea, remained prominent in Church and Imperial politics for a number of centuries. Nicea was chosen as the site of what became the First Ecumenical Council because it was near Constantine’s residence at Nicomedia. It was at Constantine’s residence near Nicomedia that Eusebius of Nicomedia baptized Constantine in 337.
After the area was conquered by the Ottoman Turks in 1326 the name of the city was changed to Izmit.