Byzantium took on the name of Konstantinoupolis ("city of Constantine", Constantinople) after its re-foundation under Roman emperor Constantine I, who transferred the capital of the Roman Empire from Rome to Byzantium in 330 AD and designated his new capital officially as Nova Roma 'New Rome'.

Constantine had altogether more colourful plans. Having restored the unity of the Empire, and being in course of major governmental reforms as well as of sponsoring the consolidation of the Christian church, Constantine was well aware that Rome was an unsatisfactory capital. Rome was too far from the frontiers, and hence from the armies and the imperial courts, and it offered an undesirable playground for disaffected politicians. Yet it had been the capital of the state for over a thousand years, and it might have seemed unthinkable to suggest that the capital be moved to a different location. Nevertheless, Constantine identified the site of Byzantium as the right place: a place where an emperor could sit, readily defended, with easy access to the Danube or the Euphrates frontiers, his court supplied from the rich gardens and sophisticated workshops of Roman Asia, his treasuries filled by the wealthiest provinces of the Empire.

Constantinople was built over 6 years, and consecrated on 11 May 330.

More Info: en.wikipedia.org