The Oxford Thesaurus An A-Z Dictionary of Synonyms - delusionn.

1) deception, trick, stratagem, artifice, ruse, pretence: It was a snare and a delusion to represent the painting as genuine. 2 false or mistaken impression, fallacy, illusion, mistake, error, misconception, misbelief, hallucination: He suffers under the delusion that he is a great pianist.