A delusion is an irrational, often bizarre thought. One of the most common types is the paranoid delusion, in which the sufferer believes that he or she is being persecuted, often by law enforcement or intelligence agencies. Delusions of reference, or the belief that one is receiving messages from public sources, are also common. For example, a person suffering from a delusion of reference may believe that a television newscaster's words contain a special message directed specifically at him. Delusions of grandeur include beliefs that one is very powerful or famous, as seen in A Beautiful Mind. Such delusions may include beliefs that one is furthering the goals of an extraterrestrial or occult power.
An example of a non-psychological delusion is the delusion employed by the Lamestream Media.