Delusion n.
Etymology: [L. delusio, fr. deludere. See Delude.]
1. The act of deluding; deception; a misleading of the mind. Pope.
2. The state of being deluded or misled.
3. That which is falsely or delusively believed or propagated; false belief; error in belief. And fondly mourned the dear delusion gone. Prior.
Syn. -- Delusion, Illusion. These words both imply some deception practiced upon the mind. Delusion is deception from want of knowledge; illusion is deception from morbid imagination. An illusion is a false show, a mere cheat on the fancy or senses. It is, in other words, some idea or image presented to the bodily or mental vision which does not exist in reality. A delusion is a false judgment, usually affecting the real concerns of life. Or, in other words, it is an erroneous view of something which exists indeed, but has by no means the qualities or attributes ascribed to it. Thus we speak of the illusions of fancy, the illusions of hope, illusive prospects, illusive appearances, etc. In like manner, we speak of the delusions of stockjobbing, the delusions of honorable men, delusive appearances in trade, of being deluded by a seeming excellence. "A fanatic, either religious or political, is the subject of strong delusions; while the term illusion is applied solely to the visions of an uncontrolled imagination, the chimerical ideas of one blinded by hope, passion, or credulity, or lastly, to spectral and other ocular deceptions, to which the word delusion is never applied." Whately.
Shades of meaning
A delusion is a mistaken conviction, an illusion a mistaken perception or inference. An illusion may be wholly of the senses; a delusion always involves some mental error. In an optical illusion the observer sees either what does not exist, or what exists otherwise than as he sees it, as when in a mirage distant springs and trees appear close at hand. We speak of the illusions of fancy or of hope, but of the delusions of the insane. A hallucination is a false image or belief which has nothing, outside of the disordered mind, to suggest it; as, the hallucinations of delirium tremens. Compare deception; insanity.