A conspiracy is a secret agreement by people to commit something wrong or illegal. Depending on the circumstances, a conspiracy may be a crime, or a civil wrong.
A “conspiracy theory” is an explanation of an event or situation that invokes an unwarranted conspiracy. (source)
A cabal is a group of people united in some close design together, usually to promote their private views or interests in an ideology, state, or other community, often by intrigue, usually unbeknown to persons outside their group. The use of this term usually carries strong connotations of shadowy corners, back rooms and insidious influence. The term is frequently used in conspiracy theories. (SOURCE)
A coup d’état; French: also known simply as a coup or an overthrow, is a type of revolution, where the illegal and overt seizure of a state by the military or other elites within the state apparatus occurs. (SOURCE)