A usurper is an illegitimate or controversial claimant to power, often but not always in a monarchy. In other words, a person who takes power of a country, city, or established region for themselves without any formal or legal right to claim it as their own. The term can be used to describe both those who overtake a region by often unexpected physical force, as well as individuals or organizations who overtake a region through political influence and subterfuge—though the word "usurper" denotes a single person; either an individual who acted alone, or the leader of a group which supported their controversial claim.
This may include a person who succeeds in establishing themselves as a monarch without inheriting the throne or any other person exercising authority to validate the claim. It may also be applied to an official acting ultra vires, outside their authority or jurisdiction.