delusion

Concise Oxford English Dictionary © 2008 Oxford University Press:
delusion/dɪˈluːʒn/

▶noun

• 1 an idiosyncratic belief or impression that is not in accordance with a generally accepted reality.

• 2 the action of deluding or the state of being deluded.

– derivatives
delusional adjective,
delusive adjective,
delusory adjective.