Usurper
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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 Henry IV of England, Miguel of Portugal, Habibullah Ghazi of Afghanistan and Barak Hussein Obama
See also