The term empire derives from the Latin imperium (power, authority). Politically, an empire is a geographically extensive group of states and peoples (ethnic groups) united and ruled either by a monarch (emperor, empress) or an oligarchy.
Aside from the traditional usage, the term empire can be used in an extended sense to denote a large-scale business enterprise (e.g. a transnational corporation), or a political organisation of either national-, regional- or city scale, controlled either by a person (a political boss) or a group authority (political bosses).
An imperial political structure is established and maintained in two ways: (i) as a territorial empire of direct conquest and control with force (direct, physical action to compel the emperor's goals) or (ii) as a coercive, hegemonic empire of indirect conquest and control with power (the perception that the emperor can physically enforce his desired goals). The former provides greater tribute and direct political control, yet limits further expansion because it absorbs military forces to fixed garrisons. The latter provides less tribute and indirect control, but avails military forces for further expansion. Territorial empires (e.g. the Mongol Empire, the Median Empire) tended to be contiguous areas. The term on occasion has been applied to maritime empires or thalassocracies, (e.g. the Athenian and British Empires) with looser structures and more scattered territories.