A usurper is an illegitimate or controversial claimant to power, often but not always in a monarchy. In other words, a person who takes the power of a country, city, or established region for himself, without any formal or legal right to claim it as his own. Usurpers are both those who overtake a region by often unexpected physical force, as well as individuals or organizations who overtake a region through political influence, machination, and subterfuge — though the word "usurper" denotes a single person; either an individual who acted alone, or the leader of a group which supported his controversial claim.
This may include a person who succeeds in establishing himself 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.