The Roman Empire is the phase of the ancient Roman civilisation characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean. The term is normally used to describe the Roman state after the establishment of rule by emperors, but it is sometimes used to refer to the expansionary Roman state both after and before the time of the first emperor, Augustus.
Principate v. Dominate
Many scholars refer to the time of Augustus and afterwards as the Principate after the title of the head of state, the Princeps, or "First Citizen". They also delineate that time from the period under Diocletian and afterwards, called the Dominate, referring to the near-supreme and despotic power of the head of state. Some, however, fail to see clearly marked boundaries or major distinctions between the earlier Principate and the later Dominate.