/ ɪˈluːʒn; NAmE / noun
1.
[ C , U ] a false idea or belief, especially about sb or about a situation :
I have no illusions about her feelings for me (= I know the truth is that she does not love me) .
She's under the illusion that (= believes wrongly that) she'll get the job.
He could no longer distinguish between illusion and reality.
2.
[ C ] something that seems to exist but in fact does not, or seems to be sth that it is not :
Mirrors in a room often give an illusion of space.
The idea of absolute personal freedom is an illusion.
—picture at optical illusion
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WORD ORIGIN
Middle English (in the sense deceiving, deception ): via Old French from Latin illusio(n-) , from illudere to mock, from in- against + ludere play.