imperium

Imperium, imperii is neuter, second declension, from impero, "I give orders". The genitive singular appears as both imperii and the contracted imperi in verse.

The word starts as a legal power, not a place. Imperium was the right to command an army and enforce orders, held by consuls, praetors and dictators, its scope marked by the number of lictors walking in front of you with the fasces. Only later, and mostly under the emperors, does it slide toward "empire" as territory, and translating every classical imperium that way flattens the point of half the political writing. Potestas is the lesser, civil authority; imperium is the one that comes with an axe. English empire, imperial and imperious follow it.