conspiracy theory [ kuhn-spir-uh-see theer-ee ] noun 1.a theory that rejects the standard explanation for an event and instead credits a covert group or organization with carrying out a secret plot. 2. a belief that a particular unexplained event was caused by such a covert group. 3. the idea that many important political events or economic and social trends are the products of deceptive plots that are largely unknown to the general public.