What is the X in X-mas?
mid-14c., "act of deception," from Old French illusion "a mocking, deceit, deception" (12c.), from Latin illusionem (nominative illusio) "a mocking, jesting, irony," from illudere "mock at," literally "to play with," from assimilated form of in- "at, upon" (see in- (2)) + ludere "to play" (see ludicrous). Sense of "deceptive appearance" developed in Church Latin and was attested in English by late 14c. Related: Illusioned "full of illusions" (1920).
illusion il·lu·sion (ĭ-lōō'zhən)
An erroneous perception of reality.
An erroneous concept or belief.
The condition of being deceived by a false perception or belief.
Something, such as a fantastic plan or desire, that causes an erroneous belief or perception.
a misrepresentation of a "real" sensory stimulus-that is, an interpretation that contradicts objective "reality" as defined by general agreement. For example, a child who perceives tree branches at night as if they are goblins may be said to be having an illusion. An illusion is distinguished from a hallucination, an experience that seems to originate without an external source of stimulation. Neither experience is necessarily a sign of psychiatric disturbance, and both are regularly and consistently reported by virtually everyone.