From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
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]