Delusion is defined as a firm belief you have that is actually not grounded in reality or truth, often occurring as a result of a mental problem.(noun)
When you hear voices in your head and believe they are the voice of the President or the voice of Jesus communicating personally with you, this is an example of a delusion.
See delusion in Webster's New World College Dictionary
Origin: ME delusioun < LL delusio < delusus, pp. of deludere
See delusion in American Heritage Dictionary 4
Origin: Middle English delusioun
Origin: , from Latin dēlūsiō, dēlūsiōn-
Origin: , from dēlūsus
Origin: , past participle of dēlūdere, to delude; see delude.
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