Valerius Licinianus Licinius was Roman emperor from 308 to 324. For most of his reign, he was the colleague and rival of Constantine I, with whom he co-authored the Edict of Milan that granted official toleration to Christians in the Roman Empire. He was finally defeated at the Battle of Chrysopolis, and was later executed on the orders of Constantine I.

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.