What is Usurper?

Legal Definition
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.
-- Wikipedia
Legal Definition
Government. One who assumes the right of government by force, contrary to and in violation of the constitution of the country. Toull. Dr. Civ. n. 32. Vide Tyranny,
-- Bouviers Law Dictionary
Legal Definition
One who assumes the right of government by force, contrary to and in violation of the constitution of the country.
-- Black's Law Dictionary