delusion, in psychology, a rigid system of beliefs with which a person is preoccupied and to which the person firmly holds, despite the logical absurdity of the beliefs and a lack of supporting evidence. Delusions are symptomatic of such mental disorders as paranoia, schizophrenia, and major depression and of such physiological conditions as senile psychosis and delirium. They vary in intensity, extent, and coherence and may represent pathological exaggeration of normal tendencies to rationalization, wishful thinking, and the like. Among the most common are delusions of persecution and grandeur; others include delusions of bodily functioning, guilt, love, and control.