delusion

US /dɪˈluː.ʒən/
UK /dɪˈluː.ʒən/
"delusion" picture
1.

a belief that is not true and is not based on reality, especially as a symptom of mental illness

:
He suffers from the delusion that he is a famous rock star.
It was a common delusion among the patients.
2.

the state of being deceived or misled

:
He was under the delusion that he could fly.
Living in a constant state of delusion can be dangerous.