in·san·i·ty (n-sn-t) n. pl. in·san·i·ties 1. Mental illness or derangement. No longer in scientific use. 2. Law a. Unsoundness of mind sufficient in the judgment of a civil court to render a person unfit to maintain a contractual or other legal relationship or to warrant commitment to a mental health facility. b. In most criminal jurisdictions, a degree of mental malfunctioning sufficient to relieve the accused of legal responsibility for the act committed. 3. a. Extreme foolishness; folly. b. Something that is extremely foolish. Synonyms: insanity, lunacy, madness, mania, dementia These nouns denote conditions of serious mental disability. Insanity is a grave, often prolonged condition that prevents a person from being held legally responsible for his or her actions: was judged not guilty for reasons of insanity. Lunacy often denotes derangement relieved intermittently by periods of clear-mindedness: yelled wildly in a moment of utter lunacy. Madness often stresses the violent aspect of mental illness: a story about obsession and madness. Mania refers principally to the excited, or manic, phase of bipolar disorder: prescribed drugs to control the patient's periods of mania. Dementia implies mental deterioration brought on by an organic brain disorder: underwent progressive stages of dementia.