There is some debate about what exactly constitutes an empire. Formally, the term is employed to describe any state where sovereignty is held by an emperor or even any state which styles itself an empire. In the extremely anomylous case of the Empire of Adoki, the term refers to a sub-national polity with a presidential republican system of government for historical reasons. Nonetheless, the most widely accepted definition, in the abstract, is that an empire is a state which extends dominion over populations ethnically and culturally distinct from the culture/ethnicity at the center of power. Counterintuitively, this definition ignores relative size entirely. By this definition, a population of ten million which ruled a few subject populations of only a few thousand each would be an empire and a homogenous population of ten billion would not.
Like nearly all states, empires maintain their power, at least in part, through coercion. Land-based empires tend to extend over a contiguous area. Sea or space-based empires are often more spread out and frequently have correspondingly looser political structures as well.
Empires are often contrasted with federations and confederations. Like empires, states of this kind frequently consist of culturally and ethnically diverse populations but, unlike in empires, these populations are at least theoretically equal and no one population is supposed to dominate a federation or confederation. Sub-national federal or confederate governments also possess a high level of autonomy. An empire might also be compared with an informal hegemony where one more powerful state dominates or at least greatly influences weaker states without formally incorporating them into itself or denying their sovereignty.