| ICD9 = | }} A delusion's commonly defined as a fixed false belief and's used in everyday language to describe a belief that's either false, fanciful or derived from deception. In psychiatry, the definition's necessarily more precise and implies that the belief's pathological (the result of an illness or illness process). As a pathology it's distinct from a belief based on false or incomplete information or certain effects of perception which would more properly be termed an apperception or il… (More on Delusion)