Roman emperor, born in Dacia, of lowly parentage; rose from a common soldier to be the son-in-law of the Emperor Diocletian, who in 292 raised him to the dignity of a Cæsar; in 305, on the death of Diocletian, he became head of the Eastern Empire, which he continued to be till his death in 311; his name is associated with a cruel persecution of the Christians under Diocletian.