Definition of "delusion"

delusion

/dɪˈl(j)uːʒ(ə)n/

noun
countable and uncountable, plural delusions
1. A false belief that is resistant to confrontation with actual facts.
2. The state of being deluded or misled, or process of deluding somebody.
3. That which is falsely or delusively believed or propagated; false belief; error in belief.
  Quotations
  Hess, always a muddled man though not so doltish as Rosenberg, flew on his own to Britain under the delusion that he could arrange a peace settlement.
  1960, William L. Shirer, The Rise and Fall of the Third Reich: A History of Nazi Germany, New York: Simon & Schuster, LCCN 81101072, page 835
4. A fixed, false belief, that will not change, despite evidence to the contrary.