The Nuttall Encyclopedia
Gallienus, Publius Licinius
Roman Emperor from 260 to 268, and for seven years (253-260) associated in the government with his father, the Emperor Valerian; under his lax rule the empire was subjected to hostile inroads on all sides, while in the provinces a succession of usurpers, known as the Thirty Tyrants, sprang up, disowning allegiance, and aspiring to the title of Cæsar; in his later years he roused himself to vigorous resistance, but in 268 was murdered by his own soldiers whilst pressing the rebel Aureolus at the siege of Milan.
Wood, James, ed. Entry for 'Gallienus, Publius Licinius'. The Nuttall Encyclopedia. https://www.studylight.org/encyclopedias/eng/nut/g/gallienus-publius-licinius.html. Frederick Warne & Co Ltd. London. 1900.