"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.