Usurper (lat. usurpare = to seize for use, to use) is a derogatory term used to describe either an illegitimate or controversial claimant to the power; often, but not always in a monarchy, or a person who succeeds in establishing himself as a monarch without inheriting the throne, or any other person exercising authority unconstitutionally. It may also be applied to an official acting ultra vires, outside his authority or jurisdiction.
Some famous examples considered usurpers are Pope Leo IX, Henry IV of England, Catherine II of Russia, Miguel of Portugal and Habibullah Ghazi of Afghanistan.