Marcus Opellius Macrinus was Roman emperor from April 217 to June 218, reigning jointly with his young son Diadumenianus. As a member of the equestrian class, he became the first emperor who did not hail from the senatorial class and also the first emperor who never visited Rome during his reign. Before becoming emperor, Macrinus served under Emperor Caracalla as a praetorian prefect and dealt with Rome's civil affairs. He later conspired against Caracalla and had him murdered in a bid to protect his own life, succeeding him as emperor.

The Roman emperor was the ruler and monarchical head of state of the Roman Empire, starting with the granting of the title augustus to Octavian in 27 BC. The term "emperor" is a modern convention, and did not exist as such during the Empire. Often when a given Roman is described as becoming emperor in English, it reflects his taking of the title augustus and later basileus. Another title used was imperator, originally a military honorific, and caesar, originally a surname. Early emperors also used the title princeps alongside other Republican titles, notably consul and pontifex maximus.