Roman emperor, born c.332;died Dadastana, Bithynia, 364. He was captain in the imperial bodyguard of the army, which was warring against Persia, and was proclaimed emperor at the death of Julian the Apostate, 363. In order to protect his army from further molestation in its retreat to the right bank of the Tigris, he relinquished the four satrapies east of the Tigris. A zealous and orthodox Christian, he reintroduced the religious toleration proclaimed by Constantine in his Milan Edict, 313.