An objective, visually convincing but unreal appearance, invariably in fantasy generated by Magic. For example, when the hero and heroine of William Morris's The Well at the World's End (1895) are protected by the Sage's Spell of illusion, their pursuers see them only as "great grey stones". Related terms are: Delusion, where things are correctly seen but misinterpreted owing to distorting obsessions or Maggots; Glamour, one person's usually self-generated illusion of Disguise, beauty or Invisibility; and Hallucination, which is subjective.
Fantasy illusions fall into two broad categories. Some are merely visual, like the glorious court of Faerie which proves to be a mere field or mound, or Mara's illusions in Roger Zelazny's Lord of Light (1967), which cause disorientation and vertigo but no direct injury. Others may be as dangerous as solid things unless properly perceived as illusion – i.e., seen through. The illusory obstacles in L Sprague de Camp's and Fletcher Pratt's Wall of Serpents (1960) must each be solved by Perception of the underlying Reality: the eponymous Serpent-wall is deadly until understood as a hedge of lingonberries. In Piers Anthony's Xanth, the sorceress Iris's powers of illusion have various logical ramifications: e.g., she can project an image of herself accompanied by an illusory voice, and thus communicate over great distances. [DRL]
see also: Trompe-L'oeil.