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 Persian Empire) 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.