insanity. n. 1) mental illness of such a severe nature that a person cannot distinguish fantasy from reality, cannot conduct her/his affairs due to psychosis, or is subject to uncontrollable impulsive behavior. Insanity is distinguished from low intelligence or mental deficiency due to age or injury. If a complaint is made to law enforcement, to the district attorney, or to medical personnel that a person is evidencing psychotic behavior, he/she may be confined to a medical facility long enough (typically 72 hours) to be examined by psychiatrists who submit written reports to the local superior/county/district court.