illusions and dellusions

il·lu·sion, noun: 1. something with deceptive appearance: something that deceives the senses or mind, e.g. by appearing to exist when it does not or appearing to be one thing when it is in fact another; 2. false idea: a false idea, conception, or belief about somebody or something; 3. deceptive power of appearances: the ability of appearances to deceive the mind and senses, or the capacity of the mind and senses to be deceived by appearances; 4. mistaken sensory perception: a misinterpretation of an experience of sensory perception, especially a visual one, where the stimuli are objectively present and the mistaken perception is due to physical rather than psychological causes.

de·lu·sion, noun: 1. false belief: a persistent false belief held in the face of strong contradictory evidence, especially as a symptom of a psychiatric condition; 2. mistaken notion: a false or mistaken belief or idea about something.