Macrinus (Marcus Opellius Severus Macrinus) (məkrī´nəs), 164–218, Roman emperor (217–18). A Moorish officer, prefect of the Praetorian Guard under Caracalla, he was threatened by the emperor's murderous plans. Macrinus therefore had the emperor killed and became the first emperor who was not a senator. He himself lost the favor of the soldiers, who revolted. He was killed, and Heliogabalus succeeded.