[1]
Note that I will differentiate between illusiond (being the old notion of discrepancy between object and percept) and illusionm (the manifest illusion that is perceived rather than inferred with the help of physics text books). I will only refer to illusionm as illusion, whereas I will refer to illusiond as mere error or discrepancy. See also the related distinction between phenomenally opaque and phenomenally transparent illusions (e.g., Metzinger 2003a, 2003b). My distinction between illusiond and illusionm is meant to be merely perceptual.
[2]
Note, that a discrepancy between stimulus and percept is necessary but not sufficient for an illusionm. Thus, all illusions require an illusiond but will only become illusionsm in some cases. My distinction is capable of sorting out illusions as relevant to perceptual psychology, it does, however, not speak to the question of how we can describe the physical stimulus in the first place, i.e., the grand illusion argument (see http://www.imprint.co.uk/books/noe.html).