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Definition of illusion : 1. A plain, delicate lace, usually of silk, used for veils, scarfs, dresses, etc.
2. A sensation originated by some external object, but so modified as in any way to lead to an erroneous perception; as when the rolling of a wagon is mistaken for thunder.
3. An unreal image presented to the bodily or mental vision; a deceptive appearance; a false show; mockery; hallucination.
4. Hence: Anything agreeably fascinating and charning; enchantment; witchery; glamour.

Synonyms:

misrepresentation, reverie, castle in the air, deceit, prank, trick, conjury, dissembling, fiction, vision, pipe dream, fantasy, gloss, false impression, head game, likeness, semblance, real, whoremaster, colour, ignis fatuus, deception, put-on, bubble, figment, antic, fast one, whoremonger, image, make-believe, delusion, fondness, dream, partiality, conjuring, conjuring trick, fallacy, confusion, color, misunderstanding, caper, legerdemain, daydream, phantasy, fancy, incantation, john, error, rainbow, magic, phantasma, mistake, joke, hallucination, thaumaturgy, psychotic belief, dissimulation, mirage, magic trick, conjuration, invocation

daemon, werewolf, apparition, bugbear, succubus, horror, myth, Frankenstein, shadow, ghoul, specter, chimera, incubus, Dracula, bogey man, nightmare, monster, phantasm, phantom, spirit, ogre, ghost, terror, bugaboo, hobgoblin, vampire

Usage examples:

• This illusion of yours about lost plans-" " Illusion?" - "Out of the Primitive", Robert Ames Bennet.
• Thy liberty, thy power, reach exactly as far as thy natural forces, and no further; all the rest is slavery and illusion. - "Rousseau Volumes I. and II.", John Morley.
• He was under no sort of illusion on that score. - "The Sign of the Spider", Bertram Mitford.