‘Delusion’, says the Concise Oxford Dictionary, is a process that entails elements of ‘imposing or being imposed on’; a ‘false impression’ or an unfounded ‘opinion’ There is a distinct feeling of malaise. So what about `illusions’, with which they are often confounded? These are supposed to have an underlying physical stimulus, though the dictionary suggests that here also `deception’of some kind, if not the very same delusive process, may be at work. 
Demonologists of the Renaissance – generally much less enlightened or humane than one would have expected – subtly distinguish the male incubus from his female counterpart (succubus). The former derives etymologically from incubate (‘to lie down’), while the latter is a derivation of succubare (‘to lie under’).
The archetypal midnight hag on her broomstick has a comic Disney touch about her, a fact which did not always escape earlier students who were not above lampooning it. But at one time she was a grim reality, even if there was the occasional judge who ruled that nocturnal flights were not illegal. We are talking about the supposed phenomenon of ‘transvection’, which is closely related, if at all distinguishable, to a whole variety of other subjects (no less controversial) for which there is yet reasonably good evidence.