con·spir·a·cy (kən-spîr'ə-sē) n., pl. -cies. 1. An agreement to perform together an illegal, wrongful, or subversive act. 2. A group of conspirators. 3. Law. An agreement between two or more persons to commit a crime or accomplish a legal purpose through illegal action. 4. A joining or acting together, as if by sinister design: a conspiracy of wind and tide that devastated coastal areas.