Illusion

n.
  1. An unreal image presented to the bodily or mental vision; a deceptive appearance; a false show; mockery; hallucination.
  2. Hence: Anything agreeably fascinating and charming; enchantment; witchery; glamour.
  3. (Physiol.) A sensation originated by some external object, but so modified as in any way to lead to an erroneous perception; as when the rolling of a wagon is mistaken for thunder.
  4. A plain, delicate lace, usually of silk, used for veils, scarfs, dresses, etc.

Some modern writers distinguish between an illusion and hallucination, regarding the former as originating with some external object, and the latter as having no objective occasion whatever.