An 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 themselves, without any formal or legal right to claim it as their own.  Usurpers can rise to power in a region by often unexpected physical force, as well as through political influence and deceit. One tactic to deter or defeat usurpation is civilian-based defense.
The word originally came from the Latin word usurpare ("to seize" or "to use").
The Greeks had their own conception of what a usurper was, calling them tyrants. In the ancient Greek usage, a tyrant (tyrannos in Greek) was an individual who rose to power via unconstitutional or illegitimate means, usually not being an heir to an existing throne. Such individuals were perceived negatively by political philosophers such as Socrates, Plato and Aristotle.
Usurpers often try to legitimize their position by claiming to be a descendant of a ruler that they may or may not be related to.