delusion

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The act or process of deluding.
• noun The state of being deluded.
• noun A false belief or opinion.
• noun Psychiatry A false belief or perception that is a manifestation of a mental illness.

from The Century Dictionary.

• noun The act of deluding; a misleading of the mind; deception.
• noun The state of being deluded; false impression or belief; error or mistake, especially of a fixed nature: as, his delusion was unconquerable. See the synonyms below.
• noun Synonyms Illusion, Delusion, Hallucination. As now technically used, especially by the best authorities in medical jurisprudence, illusion signifies a false mental appearance or conception produced by an external cause acting through the senses, the falsity of which is capable of detection by the subject of it by examination or reasoning. Thus, a mirage, or the momentary belief that a reflection in a mirror is a real object, is an illusion. A delusion is a fixed false mental conception, occasioned by an external object acting upon the senses, but not capable of correction or removal by examination or reasoning. Thus, a fixed belief that an inanimate object is a living person, that all one's friends are conspiring against one, that all food offered is poisoned, and the like, are delusions. A hallucination is a false conception occasioned by internal condition without external cause or aid of the senses, such as imagining that one hears an external voice when there is no sound to suggest such an idea. If a person walking at twilight, seeing a post, should believe it to be a spy pursuing him, and should imagine he saw it move, this would be an illusion; a continuous belief that every person one sees is a spy pursuing one, if such as cannot be removed by evidence, is a delusion; a belief that one sees such spies pursuing, when there is no object in sight capable of suggesting such a thought, is a hallucination. Illusions are not necessarily indications of insanity; delusions and hallucinations, if fixed, are. In literary and popular use an illusion is an unreal appearance presented in any way to the bodily or the mental vision; it is often pleasing, harmless, or even useful. The word delusion expresses strongly the mental condition of the person who puts too great faith in an illusion or any other error: he “labors under a delusion.” A delusion is a mental error or deception, and may have regard to things actually existing, as well as to illusions. Delusions are ordinarily repulsive and discreditable, and may even be mischievous. We speak of the illusiom of fancy, hope, youth, and the like, but of the delusions of a fanatic or a lunatic. A hallucination is the product of an imagination disordered, perhaps beyond the bounds of sanity; a flighty or crazy notion or belief, generally of some degree of permanence; a special aberration of belief as to some specific point: the central suggestion in the word is that of the groundlessness of the belief or opinion.

from the GNU version of the Collaborative International Dictionary of English.

• noun The act of deluding; deception; a misleading of the mind.
• noun The state of being deluded or misled.
• noun That which is falsely or delusively believed or propagated; false belief; error in belief.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A false belief that is resistant to confrontation with actual facts.
• noun The state of being deluded or misled.
• noun That which is falsely or delusively believed or propagated; false belief; error in belief.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a mistaken or unfounded opinion or idea
• noun the act of deluding; deception by creating illusory ideas
• noun (psychology) an erroneous belief that is held in the face of evidence to the contrary