"Roman Empire" is also used as translation of the expression, Imperium Romanum, probably the best known Latin expression where the word imperium is used in the meaning of a territory, the "Roman Empire", as that part of the world under Roman rule.
The expansion of this Roman territory beyond the borders of the initial city-state of Rome had started long before the state organization turned into an Empire. In its territorial peak after the conquest of Dacia by Trajan, the Roman Empire controlled approximately 5.
900 000 km². (2,300,000 sq.mi.) of land surface, thereby being the largest of all empires during the classical antiquity period of European history.
In the centuries before the autocracy of Augustus, Rome (Roman Kingdom and Roman Republic) had already accumulated most of its territory beyond the Italian Peninsula, including former Mediterranean competitors Syracuse and Carthage. In the late Republic, Augustus definitively added Egypt to the Imperium Romanum.