ILLUSION

Definition of “ILLUSION” from the Oxford English Dictionary

Sensuous perception of an external object, involving a false belief or conception: strictly distinguished from hallucination, but in general use often made to include it, and hence = the apparent perception of an external object when no such object is present, or of attributes of an object which do not exist.
  the argument that the objects of sense-experience, usually called ideas, appearances, or sense-data, cannot be objects in a physical world independent of the Perceiver, since they vary according to his condition and environment.