An empire is a multi-ethnic or multinational state with political and/or military dominion of populations who are culturally and ethnically distinct from the imperial (ruling) ethnic group and its culture. This is in contrast to a federation, which is an extensive state voluntarily composed of autonomous states and peoples. An empire is a large polity which rules over territories outside of its original borders.
Definitions of what physically and politically constitute an empire vary. It might be a state affecting imperial policies or a particular political structure. Empires are typically formed from diverse ethnic, national, cultural, and religious components. 'Empire' and 'colonialism' are used to refer to relationships between powerful state or society versus a less powerful one.
Tom Nairn and Paul James define empires as polities that "extend relations of power across territorial spaces over which they have no prior or given legal sovereignty, and where, in one or more of the domains of economics, politics, and culture, they gain some measure of extensive hegemony over those spaces for the purpose of extracting or accruing value". Rein Taagepera has defined an empire as "any relatively large sovereign political entity whose components are not sovereign".
Sometimes, an empire is a semantic construction, such as when a ruler assumes the title of "emperor". That ruler's nation logically becomes an "empire", despite having no additional territory or hegemony. Examples of this form of empire are the Central African Empire, or the Korean Empire proclaimed in 1897 when Korea, far from gaining new territory, was on the verge of being annexed by the Empire of Japan, the last to use the name officially. Among the last of the empires in the 20th century were the Central African Empire, Ethiopia, Vietnam, Manchukuo, the German Empire, and Korea.
The terrestrial empire's maritime analogue is the thalassocracy, an empire composed of islands and coasts which are accessible to its terrestrial homeland, such as the Athenian-dominated Delian League.
Furthermore, empires can expand by both land and sea. Stephen Howe notes that empires by land can be characterized by expansion over terrain, "extending directly outwards from the original frontier" while an empire by sea can be characterized by colonial expansion and empire building "by an increasingly powerful navy".