The Roman Empire is the term conventionally used to describe the Ancient Roman polity in the centuries following its reorganization under the leadership of Augustus in late 1st century BC. After Constantinople had been made capital and the Western parts were lost, the Eastern part continued its existence, in what is currently known as Byzantine Empire.
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 sq.km. (2,300,000 sq.mi.) of land surface, thereby being the largest of all empires during the classical antiquity period of European history.