In psychological research, conspiracy theories are often defined as explanations of events involving the hidden action of a malevolent group. Such a definition raises a false negative problem, as it does not capture conspiracy theories that are not about events. It also raises a false positive problem because it categorises any conspiracy-based explanation as a conspiracy theory, even though distinguishing conspiracy theories from other conspiracy claims is at the core of many attempts to define this notion. Based on more elaborated definitions and a conceptual reengineering approach, we propose that conspiracy theories can be defined as claims that the public is being pervasively lied to regarding some aspect(s) of reality, to allow some group(s) to enact a harmful, self-serving agenda. Compared to other definitions, ours has the advantage of not taking a position regarding the truth value of conspiracy theories, making it highly operative for psychological research.