A coup d'état, often just coup, is a quick and illegal overthrow of an existing government by a small group or "terrorists" or "resistance fighters", even "freedom fighters", depending of the ideological stance of reporting or the origin of political propaganda.
It is often contrasted with a revolution, claimed to be by larger numbers of people working to achieve more fundamental changes. Leftists may try associate coups with right-wing politics, revolutions / uprisings with left-wing politics. However, while leftists have often advocated radical changes, this distinction is dubious, as Communists, typically lacking majority support, have often gained power in coups (or through foreign military interventions), while instead the fall of Communism had broad popular support and thus can be considered a revolution or revolutions.
The term is French for "a (sudden) blow (or strike) to a state" (literally: coup = hit and état = state). The term coup can also be used in a casual sense to mean a gain in advantage of one nation or entity over another; e.g. an intelligence coup. By analogy, the term is also applied to corporations, etc; e.g. a boardroom coup.
Tactically, a coup usually involves control of some active portion of the military while neutralizing the remainder of a country's armed services. This active group captures or expels leaders, seizes physical control of important government offices, means of communication, and the physical infrastructure, such as streets and power plants. The coup succeeds if its opponents fail to dislodge the plotters, allowing them to consolidate their position, obtain the surrender or acquiescence of the populace and surviving armed forces, and claim legitimacy. Coups typically use the power of the existing government for its own takeover.
Since the unsuccessful coup attempts of Wolfgang Kapp (de) together with Reichswehr general Walther Freiherr von Lüttwitz in 1920 (Kapp-Aufstand against the socialist dictate), and of Adolf Hitler in 1923 (March on the Feldherrnhalle), the Swiss German word "Putsch" (pronounced /pʊtʃ/; originally coined with the Züriputsch of 1839) is often used also, even in French (such as the putsch of 8 November 1942 and the putsch of 21 April 1961, both in Algiers), English (Castro's 1959 Putsch in Cuba) and Russian (August Putsch in 1991), while the direct German translation is Staatsstreich.