Source : Webster's Revised Unabridged Dictionary (1913)

Delusion \De*lu"sion\n. [L. delusio, fr. deludere. See
   {Delude}.]
   1. The act of deluding; deception; a misleading of the mind.
      --Pope.

   2. The state of being deluded or misled.

   3. That which is falsely or delusively believed or
      propagated; false belief; error in belief.

            And fondly mourned the dear delusion gone. --Prior.

   Syn: {Delusion}, {Illusion}.

   Usage: These words both imply some deception practiced upon
          the mind. Delusion is deception from want of
          knowledge; illusion is deception from morbid
          imagination. An illusion is a false show, a mere cheat
          on the fancy or senses. It is, in other words, some
          idea or image presented to the bodily or mental vision
          which does not exist in reality. A delusion is a false
          judgment, usually affecting the real concerns of life.
          Or, in other words, it is an erroneous view of
          something which exists indeed, but has by no means the
          qualities or attributes ascribed to it. Thus we speak
          of the illusions of fancy, the illusions of hope,
          illusive prospects, illusive appearances, etc. In like
          manner, we speak of the delusions of stockjobbing, the
          delusions of honorable men, delusive appearances in
          trade, of being deluded by a seeming excellence. ``A
          fanatic, either religious or political, is the subject
          of strong delusions; while the term illusion is
          applied solely to the visions of an uncontrolled
          imagination, the chimerical ideas of one blinded by
          hope, passion, or credulity, or lastly, to spectral
          and other ocular deceptions, to which the word
          delusion is never applied.'' --Whately.

Source : WordNet®

delusion
     n 1: (psychology) an erroneous belief that is held in the face of
          evidence to the contrary [syn: {psychotic belief}]
     2: a mistaken or unfounded opinion or idea; "he has delusions
        of competence"; "his dreams of vast wealth are a
        hallucination" [syn: {hallucination}]
     3: the act of deluding; deception by creating illusory ideas
        [syn: {illusion}, {head game}]