DELUSION

n. illusion, belief in something that is not true

A delusion is commonly defined as a fixed false belief  and is used in everyday language to describe a belief that is either false, fanciful or derived from deception. In psychiatry, the definition is necessarily more precise and implies that the belief is pathological (the result of an illness or illness process). As a pathology it is distinct from a belief based on false or incomplete information or certain effects of perception which would more properly be termed an apperception or illusion.
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Delusion /de·lu·sion/ an idiosyncratic false belief that is firmly maintained in spite of incontrovertible and obvious proof or evidence to the [more]Delusion - Community and Resources

Noun
1. (psychology) an erroneous belief that is held in the face of evidence to the contrary
(synonym) psychotic belief
(hypernym) psychological state, mental state
(hyponym) delusions of grandeur
(derivation) deceive, lead on, delude, cozen
(classification) psychology, psychological science
2. a mistaken or unfounded opinion or idea; "he has delusions of competence"; "his dreams of vast wealth are a hallucination"
(synonym) hallucination
(hypernym) misconception
(hyponym) disorientation, freak out
(derivation) deceive, lead on, delude, cozen
3. the act of deluding; deception by creating illusory ideas
(synonym) illusion, head game
(hypernym) deception, deceit, dissembling, dissimulation
(derivation) deceive, lead on, delude, cozen

(n.)
The state of being deluded or misled.
  

(n.)
The act of deluding; deception; a misleading of the mind.
  

(n.)
That which is falsely or delusively believed or propagated; false belief; error in belief.