criminal insanity

Definition from Nolo’s Plain-English Law Dictionary

A mental defect or disease that, as understood in most states, makes it impossible for a person to know what he or she is doing; or if he or she does know, to know that what they are doing is wrong. Some states define as insane those defendants who acted under an irresistible impulse, even if they knew their actions were wrong. Defendants who are criminally insane cannot be convicted of a crime, because criminal conduct involves the conscious intent to do wrong -- a choice that the criminally insane cannot meaningfully make. (See also: irresistible impulse test, McNaghten Rule)

Definition provided by Nolo’s Plain-English Law Dictionary.