In a political sense, conspiracy refers to a group of people united in the goal of usurping, altering or overthrowing an established political power. Typically, the final goal is to gain power through a revolutionary coup d'état or through assassination. A conspiracy can also be used for infiltration of the governing system.
A conspiracy is to be contrasted with a cabal. The two are similar but have quite different connotations; in contrast to a cabal, a conspiracy usually looks to overthrow a fixed power instead of usurping it from within.
A "conspiracy theory" is a belief that a conspiracy has actually been decisive in producing a political event which the theorists strongly disapprove of.