In political science, legitimacy is the right and acceptance of an authority, usually a governing law or a regime. Whereas authority denotes a specific position in an established government, the term legitimacy denotes a system of government—wherein government denotes "sphere of influence". An authority viewed as legitimate often has the right and justification to exercise power. Political legitimacy is considered a basic condition for governing, without which a government will suffer legislative deadlock(s) and collapse. In political systems where this is not the case, unpopular regimes survive because they are considered legitimate by a small, influential elite. In Chinese political philosophy, since the historical period of the Zhou Dynasty (1046–256 BC), the political legitimacy of a ruler and government was derived from the Mandate of Heaven, and unjust rulers who lost said mandate therefore lost the right to rule the people.

In moral philosophy, the term legitimacy is often positively interpreted as the normative status conferred by a governed people upon their governors' institutions, offices, and actions, based upon the belief that their government's actions are appropriate uses of power by a legally constituted government.

The Enlightenment-era British social philosopher John Locke (1632–1704) said that political legitimacy derives from popular explicit and implicit consent of the governed: "The argument of the [Second] Treatise is that the government is not legitimate unless it is carried on with the consent of the governed." The German political philosopher Dolf Sternberger said that "[l]egitimacy is the foundation of such governmental power as is exercised, both with a consciousness on the government's part that it has a right to govern, and with some recognition by the governed of that right". The American political sociologist Seymour Martin Lipset said that legitimacy also "involves the capacity of a political system to engender and maintain the belief that existing political institutions are the most appropriate and proper ones for the society". The American political scientist Robert A. Dahl explained legitimacy as a reservoir: so long as the water is at a given level, political stability is maintained, if it falls below the required level, political legitimacy is endangered.