delusion

Short definition from Wiktionary

Noun

1. 1
   A false belief that is resistant to confrontation with actual facts.
2. 2
   The state of being deluded or misled, or process of deluding somebody.
3. 3
   That which is falsely or delusively believed or propagated; false belief; error in belief.
4. 4
   A fixed, false belief, that will not change, despite evidence to the contrary.