A perception without objective reality

Solution (By Examveda Team)

One word substitute is Hallucination.

Allusion: an expression designed to call something to mind without mentioning it explicitly; an indirect or passing reference.
Illusion: an instance of a wrong or misinterpreted perception of a sensory experience.
Dyspepsia: indigestion.
Hallucination: an experience involving the apparent perception of something not present.