Definition: Gallienus (Publius Licinius Egnatius Gallienus), born c. A.D. 218, was a Roman emperor from 253, when he was named Caesar -- following the senate appointment of his father Valerian to Augustus -- until his murder by his own generals in 268 during the siege of Milan. Shortly after becoming Caesar, Valerian elevated him to co-emperor putting Gallienus in charge in the West. Gallienus campaigned against Goths, Germans, and barbarians along the Danube. In 259-60, Shapur I, king of the Persians, captured and killed Gallienus' father.
In 267, when the Goths attacked, Gallienus set out again to fight them. Aureolus, in charge of the cavalry in Milan, rebelled, and so Gallienus returned to handle it. It was at this point that Gallienus was murdered by a group of generals who included the next emperor, Claudius II.
Gallienus' is counted among those emperors who opposed the senatorial class, Caligula, Nero, Domitian, and Septimius Severus.
From 260-268, Gallienus debased the coinage to below 2% silver.
Sources:
Diana Bowder's Who Was Who in the Roman World.
"The Policy of the Emperor Gallienus, by Lukas de Blois. Review author: M. T. W. Arnheim. The American Historical Review, Vol. 82, No. 3. (Jun., 1977), pp. 614-615.
Examples:
The Historia Augusta writes vehemently against Gallienus. Here's an example. Gallienus, on the other hand, when he learned that Macrianus and his sons were slain, as though he were secure in his power and his father were now set free, surrendered himself to lust and pleasure.
