Usurper is a derogatory term used to describe either an illegitimate or controversial claimant to the power; often, but not always in a monarchy

Monarchy

A monarchy is a form of government in which the office of head of state is usually held until death or abdication and is often hereditary and includes a royal house. In some cases, the monarch is elected...

, or a person who succeeds in establishing himself as a monarch without inheriting the throne, or any other person exercising authority unconstitutionally. It may also be applied to an official acting ultra vires

Ultra vires

Ultra vires is a Latin phrase meaning literally "beyond the powers", although its standard legal translation and substitute is "beyond power". If an act requires legal authority and it is done with such authority, it is...

, outside his authority or jurisdiction.

Some famous examples considered usurpers are Pope Leo IX

Pope Leo IX

Pope Saint Leo IX , born Bruno of Eguisheim-Dagsburg, was Pope from February 12, 1049 to his death. He was a German aristocrat and as well as being Pope was a powerful secular ruler of central Italy. He is regarded as a saint by the Roman Catholic Church, with the feast day of April 19...

, Henry IV of England

Henry IV of England

Henry IV was King of England and Lord of Ireland . He was the ninth King of England of the House of Plantagenet and also asserted his grandfather's claim to the title King of France. He was born at Bolingbroke Castle in Lincolnshire, hence his other name, Henry Bolingbroke...

, Catherine II of Russia

Catherine II of Russia

Catherine II, also known as Catherine the Great , Empress of Russia, was born in Stettin, Pomerania, Prussia on as Sophie Friederike Auguste von Anhalt-Zerbst-Dornburg...

, Miguel of Portugal

Miguel of Portugal

Dom Miguel I, sometimes Michael , was the King of Portugal between 1828 and 1834, the seventh child and second son of King John VI and his queen, Charlotte of Spain....

 and Habibullah Ghazi of Afghanistan.