A conspiracy theory, according to Merriam-Webster, is “a theory that explains an event or set of circumstances as the result of a secret plot by usually powerful conspirators.” Alternatively, it’s “a theory asserting that a secret of great importance is being kept from the public”; the idea presumably is that insiders have conspired to keep the truth hidden.

Dictionary.com takes a similar approach: “a theory that rejects the standard explanation for an event and instead credits a covert group or organization with carrying out a secret plot” or “a belief that a particular unexplained event was caused by such a covert group.” In all these cases, a conspiracy theory requires a conspiracy to make something happen or to keep something hidden. The cabal has to be hidden and the conspirators powerful; an accusation that some people got together to plan a crime doesn’t count as a conspiracy theory unless the perpetrators are extremely rich or powerful.
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