| }} A delusion is commonly defined as a fixed false belief and is used in everyday language to describe a belief that's either false, fanciful or derived from deception. In psychiatry, the definition is necessarily more precise and implies that the belief is 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 illusio… (More on Delusion)