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top: straight horizontal rules appear curved
bottom: a gray box against a black background appears lighter than the same gray box against a white background
a. An erroneous perception of reality: Mirrors gave the illusion of spaciousness.
b. An erroneous concept or belief: The notion that money can buy happiness is an illusion.
2. The condition of being deceived by a false perception or belief: spent months flailing about in illusion.
3. Something that is erroneously perceived or construed: The animal in the shadows turned out to be an illusion.
4. A fine transparent net fabric, used for dresses or trimmings.
[Middle English illusioun, from Old French, from Late Latin illūsiō, illūsiōn-, from Latin, a mocking, irony, from illūsus, past participle of illūdere, to mock : in-, against; see in-2 + lūdere, to play; see leid- in Indo-European roots.]
il·lu′sion·al, il·lu′sion·ar′y (-zhə-nĕr′ē) adj.