noun (n)

• (psychology) an erroneous belief that is held in the face of evidence to the contrary(noun.state)

  Synonym:
  psychotic belief
  

  source: wordnet30
• a mistaken or unfounded opinion or idea(noun.cognition)

  Example:
  He has delusions of competence.
  

  Synonym:
  hallucination
  

  source: wordnet30
• the act of deluding; deception by creating illusory ideas(noun.act)

  Synonym:
  head game, illusion
  

  source: wordnet30
• The act of deluding; deception; a misleading of the mind.(noun)

  source: webster1913