The title of imperator originally meant the rough equivalent of commander under the Roman Republic. Later, it became a part of the titulature of the Roman Emperors as their praenomen. The Roman emperors themselves generally based their authority on multiple titles and positions, rather than preferring any single title. Nevertheless, imperator was used relatively consistently as an element of a Roman ruler's title throughout the Principate and the Dominate. The word itself derives from the stem of the verb imperare, meaning 'to order, to command'. The English word emperor derives from imperator via Old French: Empereür.

Gaius Julius Caesar Augustus, also known as Octavian, was the founder of the Roman Empire. He reigned as the first Roman emperor from 27 BC until his death in AD 14. The reign of Augustus initiated an imperial cult, as well as an era of imperial peace in which the Roman world was largely free of armed conflict. The Principate system of government was established during his reign and lasted until the Crisis of the Third Century.