The Collaborative International Dictionary of English v.0.48:

Delusion \De*lu"sion\n. [L. delusio, fr. deludere. See
   Delude.]
   1. The act of deluding; deception; a misleading of the mind.
      --Pope.
      [1913 Webster]

   2. The state of being deluded or misled.
      [1913 Webster]

   3. That which is falsely or delusively believed or
      propagated; false belief; error in belief.
      [1913 Webster]

            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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

188 Moby Thesaurus words for "delusion":
   aberrancy, aberration, acting, affectation, agnosia, airy nothing,
   apparition, appearance, artifice, attitudinizing, autism,
   bamboozlement, befooling, block, blocking, bluff, bluffing, bubble,
   calculated deception, casuistry, cheat, cheating, chicane,
   chicanery, chimera, circumvention, color, coloring, conning,
   counterfeit, daydream, deceit, deceiving, deception, deceptiveness,
   defectiveness, defrauding, delirium, deluded belief,
   delusion of persecution, delusiveness, dereism, deviancy, disguise,
   disorientation, dissemblance, dissembling, dissimulation,
   distortion, dream, dream vision, dreamland, dreamworld, dupery,
   eidolon, enmeshment, ensnarement, entanglement, entrapment,
   equivocation, errancy, erroneousness, error, facade, face, fake,
   fakery, faking, fallaciousness, fallacy, false air, false belief,
   false front, false show, falseness, falsity, fancy, fantasy, fault,
   faultiness, feigning, feint, figment, flaw, flawedness,
   flight of ideas, flimflam, flimflammery, fond illusion, fooling,
   four-flushing, fraud, front, ghost, gilt, gloss, hallucination,
   hallucinosis, hamartia, heresy, heterodoxy, hoodwinking, humbug,
   humbuggery, ignis fatuus, illusion, imposture, kidding, masquerade,
   mental block, mental confusion, meretriciousness, mirage,
   misapplication, misbelief, misconception, misconstruction,
   misdoing, misfeasance, misinterpretation, misjudgment, mistake,
   nihilism, nihilistic delusion, ostentation, outward show,
   outwitting, overreaching, paralogia, peccancy, perversion,
   phantasm, phantom, pipe dream, playacting, pose, posing, posture,
   pretense, pretension, pretext, psychological block, putting on,
   representation, ruse, seeming, self-contradiction, self-deceit,
   self-deception, self-delusion, semblance, shade, sham, show,
   simulacrum, simulation, sin, sinfulness, snow job, song and dance,
   sophism, sophistry, speciousness, spoofery, spoofing, spuriousness,
   stratagem, subterfuge, swindling, trick, trickery, trickiness,
   tricking, trip, unorthodoxy, untrueness, untruth, untruthfulness,
   vapor, varnish, victimization, vision, willful misconception,
   window dressing, wishful thinking, wrong, wrong impression,
   wrongness



Bouvier's Law Dictionary, Revised 6th Ed (1856):

DELUSION, med. jurisp. A diseased state of the mind, in which persons
believe things to exist, which exist only, or in the degree they are
conceived of only in their own imaginations, with a persuasion so fixed and
firm, that neither evidence nor argument can convince them to the contrary.
     2. The individual is, of course, insane. For example, should a parent
unjustly persist without the least ground in attributing to his daughter a
course of vice, and use her with uniform unkindness, there not being the
slightest pretence or color of reason for the supposition, a just inference
of insanity, or delusion, would arise in the minds of a jury: because a
supposition long entertained and persisted in, after argument to the
contrary, and against the natural affections of a parent, suggests that he
must labor under some morbid mental delusion. 3 Addams' R. 90, 91; Id. 180;
Hagg. R. 27 and see Dr. Connolly's Inquiry into Insanity, 384; Ray, Med.
Jur. Prel. Views., Sec. 20, p. 41, and Sec. 22, p. 47; 3 Addams, R. 79; 1
Litt. R. 371 Annales d'Hygiene Publique, tom. 3, p. 370; 8 Watts, 70; 13
Ves. 89; 1 Pow. Dev. by Jarman, 130, note Shelf. on Lun. 296; 2 Bouv. Inst.
n. 2104-10.



The Devil's Dictionary (1881-1906):

DELUSION, n.  The father of a most respectable family, comprising
Enthusiasm, Affection, Self-denial, Faith, Hope, Charity and many
other goodly sons and daughters.

    All hail, Delusion!  Were it not for thee
    The world turned topsy-turvy we should see;
    For Vice, respectable with cleanly fancies,
    Would fly abandoned Virtue's gross advances.
                                                        Mumfrey Mappel