Richard has never used technical terminology in the titles of his book that he did not himself invent (The Extended Phenotype). They are all, with that exception, extremely poetic, and the words themselves cannot in any way be taken to literally describe the subject matter.
Still, Princeton on-line dictionary has these definitions:
# S: (n) delusion, psychotic belief ((psychology) an erroneous belief that is held in the face of evidence to the contrary)
# S: (n) delusion, hallucination (a mistaken or unfounded opinion or idea) "he has delusions of competence"; "his dreams of vast wealth are a hallucination"
# S: (n) delusion, illusion, head game (the act of deluding; deception by creating illusory ideas)
I have found a number of similar definitions, the only important deviation being those that explicitly exclude unfounded convictions of a religious nature. This is balant special pleading, and although I have not yet read the God Delusion (and will report back on the occassion that I do) it is in Dawkin's style to tackle this precise issue pretty early on in the book. It is also in Dawkin's style to explain the exact origin of his title in the preface. Finally, the book itself is intended, at least in part, to be the case that the belief in gods is itself a delusion. For that reason alone, regardless of whether or not the attempt is successful, the title "The God Delusion" is perfectly appropriate.
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