A coup d'état (/ˌkuːdeɪˈtɑː/ ^ⓘ; French: [ku deta] ^ⓘ; lit. 'stroke of state'), or simply a coup, is typically an illegal and overt attempt by a military organization or other government elites to unseat an incumbent person or leadership. A self-coup is said to take place when a leader, having come to power through legal means, tries to stay in power through illegal means.

By one estimate, there were 457 coup attempts from 1950 to 2010, half of which were successful. Most coup attempts occurred in the mid-1960s, but there were also large numbers of coup attempts in the mid-1970s and the early 1990s. Coups occurring in the post-Cold War period have been more likely to result in democratic systems than Cold War coups, though coups still mostly perpetuate authoritarianism.

Many factors may lead to the occurrence of a coup, as well as determine the success or failure of a coup. Once a coup is underway, coup success is driven by coup-makers' ability to get others to believe that the coup attempt will be successful. The number of successful coups has decreased over time. Failed coups in authoritarian systems are likely to strengthen the power of the authoritarian ruler. The cumulative number of coups is a strong predictor of future coups, a phenomenon referred to as the "coup trap".

In what is referred to as "coup-proofing", regimes create structures that make it hard for any small group to seize power. These coup-proofing strategies may include the strategic placing of family, ethnic, and religious groups in the military and the fragmenting of military and security agencies. However, coup-proofing reduces military effectiveness as loyalty is prioritized over experience when filling key positions within the military.

The term comes from French coup d'État, literally meaning a 'stroke of state' or 'blow of state'. In French, the word État (French: [eta]) is capitalized when it denotes a sovereign political entity. Although the concept of a coup d'état has featured in politics since antiquity, the phrase is of relatively recent coinage. It did not appear within an English text before the 19th century except when used in the translation of a French source, there being no simple phrase in English to convey the contextualized idea of a 'knockout blow to the existing administration within a state'.

One early use within text translated from French was in 1785 in a printed translation of a letter from a French merchant, commenting on an arbitrary decree, or arrêt, issued by the French king restricting the import of British wool. What may be its first published use within a text composed in English is an editor's note in the London Morning Chronicle, 1804, reporting the arrest by Napoleon in France, of Moreau, Berthier, Masséna, and Bernadotte: "There was a report in circulation yesterday of a sort of coup d'état having taken place in France, in consequence of some formidable conspiracy against the existing government."

In the British press, the phrase came to be used to describe the various murders by Napoleon's alleged secret police, the Gens d'Armes d'Elite, who executed the Duke of Enghien: "the actors in torture, the distributors of the poisoning draughts, and the secret executioners of those unfortunate individuals or families, whom Bonaparte's measures of safety require to remove. In what revolutionary tyrants call grand[s] coups d'état, as butchering, or poisoning, or drowning, en masse, they are exclusively employed."

A self-coup, also called an autocoup (from Spanish autogolpe) or coup from the top, is a form of coup d'état in which a political leader, having come to power through legal means, stays in power illegally through the actions of themselves or their supporters. The leader may dissolve or render powerless the national legislature and unlawfully assume extraordinary powers. Other measures may include annulling the constitution, suspending civil courts, and having the head of government assume dictatorial powers.