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The Latin term Imperium signifies first of all the civilian and military power in Rome, then the domination by the city of a vast territory, and finally the political system governed by an imperator. It allows us to return to the approaches proposed since the Age of the Enlightenment until the turning point of the twentieth and twenty-first centuries AD, both about the fate of a city-state becoming the head of a vast Mediterranean territory, and the expression of the powers within the Vrbs and what would eventually be named the Imperium Romanum. Step by step, the paper aims to be philological and historiographical, and insists in particular on the expressions of power, from the middle of the Res publica to the third century AD, as well as on the notions of empire, taking into account Roman ways of considering the relationship between the city ( Vrbs) and the explored—and subsequently controlled—world ( Orbis terrarum).