Scholars debate about what exactly constitutes an empire (from the Latin "imperium", denoting military command within the ancient Roman government). Generally, they may define an empire as a state that extends dominion over and populations distinct culturally and ethnically from the culture/ethnicity at the center of power. Other definitions may emphasize economic or political factors. The term generally implies military hegemonic power.
Like other states, an empire maintains its political structure at least partly by coercion. Land-based empires (such as the Mongol Empire or the Achaemenid Persia) tend to extend in a contiguous area; sea-borne empires, also known as thalassocracies (the Athenian, Portuguese and the British empires provide examples), may feature looser structures and more scattered territories.
Empires predate the Romans by several hundred years: Egypt, for example, set up an empire in the 16th century BC by invading and then incorporating Nubia and the ancient city-states of the Levant. The Akkadian Empire of Sargon of Akkad exists as one of the earliest models of a far-flung, land-based empire. Empire contrasts with the example of a federation, where a large, multi-ethnic state — or even an ethnically homogeneous one like Japan or a small area like Switzerland — relies on mutual agreement amongst its component political units which retain a high degree of autonomy. Additionally, one can compare physical empires with potentially more abstract or less formally structured hegemonies in which the sphere of influence of a single political unit (such as a city-state) dominates a culturally unified area politically or militarily. A second side of this same coin shows in potentially inherent tactics of divide and conquer by different factions ("the enemy of my enemy is my friend") and central intervention for the greater whole's benefit.
Compare also the concept of superpowers and hyperpowers. (Some commentators have seen the British Empire as a hyperpower, in its heyday as the largest empire in world history (covering about one-fourth of the Earth's land surface) with established political, economical, financial, and scientific hegemony over the whole world).