A delusion is "a false belief based on incorrect inference about external reality that is firmly sustained despite what almost everyone else believes and despite what constitutes incontrovertible and obvious proof or evidence to the contrary".
A hallucination is a "sensory perception that has the compelling sense of reality of a true perception but that occurs without external stimulation of the relevant sensory organ".
Delusion is, therefore, a belief, idea, or conviction firmly held despite abundant information to the contrary. The partial or complete loss of reality test is the first indication of a psychotic state or episode. Beliefs, ideas, or convictions shared by other people, members of the same collective, are not, strictly speaking, delusions, although they may be hallmarks of shared psychosis.
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase the print book, or 16 e-books, or 3 DVDs with 16 hours of video lectures on narcissists, psychopaths, and abuse in relationships: http://www.narcissistic-abuse.com/the...)