GALLIENUS, P. LICINIUS, emperor of Rome from 253 to 268 A.D. In the beginning of his reign he gave proof both of bravery and ability in his defeat of the barbarians who had invaded Italy from the north. He afterwards belied the promise of his youth, and gave himself up to excess and debauchery of every kind. Usurpers soon appeared in almost every province of the empire, and this period of Roman history came to be called the reign of the thirty tyrants. Gallienus was killed at Milan while besieging one of his refractory vassals in that city.