The Principate was the form of imperial government of the Roman Empire from the beginning of the reign of Augustus in 27 BC to the end of the Crisis of the Third Century in AD 284, after which it evolved into the Dominate. The principate was characterised by the reign of a single emperor (princeps) and an effort on the part of the early emperors, at least, to preserve the illusion of the formal continuance, in some aspects, of the Roman Republic.

The Roman Empire was the post-Republican state of ancient Rome. It is generally understood to mean the period and territory ruled by the Romans following Octavian's assumption of sole rule under the Principate in 27 BC. It included territories in Europe, North Africa, and Western Asia and was ruled by emperors. The fall of the Western Roman Empire in 476 AD conventionally marks the end of classical antiquity and the beginning of the Middle Ages.