noun

an idiosyncratic belief or impression maintained despite being contradicted by reality or rational argument, typically as a symptom of mental disorder.

the delusion of being watched

misapprehension,mistaken impression,false impression,mistaken belief,misconception,misunderstanding,mistake,error,misinterpretation,misconstruction,misbelief,fallacy,illusion,figment of the imagination,fantasy,chimera,fool’s paradise,self-deception