A usurper is an illegitimate or controversial claimant to power, often but not always in a monarchy. This may include a person who succeeds in establishing himself as a monarch without inheriting the throne or any other person exercising authority unconstitutionally. A person who takes the power of the country for himself or herself at the expense of the people. It may also be applied to an official acting ultra vires, outside his authority or jurisdiction.
References
Usurper Wikipedia(Text) CC BY-SA