Roman emperor from 375 to 383, eldest son of Valentinian I., born in Pannonia; at 16, in conjunction with his four-year-old brother, Valentinian II., became ruler over the Western Empire, and three years later found himself, by the death of his uncle Valens, head also of the Eastern Empire, a year after which he summoned Theodosius to be his colleague; his reign is noted for the stern repression of the remains of the heathen worship; in 383, while endeavouring to combat the usurper Maximus, he was captured at Lyons and there put to death (359-383).