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English definition of “illusion”

illusion

noun     /ɪˈluː.ʒən/
C2 [C or U] an idea or belief that is not true: He had no illusions about his talents as a singer. I'm under no illusions (= I understand the truth) about the man I married. My boss is labouring under the illusion that (= wrongly believes that) the project will be completed on time.Unreal things and unreality C2 [C] something that is not really what it seems to be: A large mirror in a room can create the illusion of space. The impression of calm in the office is just an illusion.Hiding and disguising
(Definition of illusion noun from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)
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