John VI (John Cantacuzene), c.1292-1383, Byzantine emperor (1347-54). He was chief minister under Andronicus III, after whose death he proclaimed himself emperor and made war on the rightful heir, John V. He was aided by the Ottoman Turks. The war allowed Stephen Dušan to build his Serbian empire. John's reign briefly quieted civil and religious strife within the empire. In 1354 he abdicated in favor of John V and retired to a monastery, where he wrote a history of the period 1320-56. A defender of the mystical theory known as Hesychasm, he was instrumental in its acceptance by the Orthodox Eastern Church.