delusion
US /dɪˈluː.ʒən/
UK /dɪˈluː.ʒən/
1.
a belief that is not true and is not based on reality, especially as a symptom of mental illness
:
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He suffers from the delusion that he is a famous rock star.
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It was a common delusion among the patients.
2.
the state of being deceived or misled
:
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He was under the delusion that he could fly.
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Living in a constant state of delusion can be dangerous.