insanity

:  the condition of being abnormal or unhealthy in mind

1dated :  a severely disordered state of the mind usually occurring as a specific disorder

2law :  unsoundness of mind or lack of the ability to understand that prevents one from having the mental capacity required by law to enter into a particular relationship, status, or transaction or that releases one from criminal or civil responsibility

1 :  unsoundness of mind or lack of the ability to understand that prevents one from having the mental capacity required by law to enter into a particular relationship, status, or transaction or that releases one from criminal or civil responsibility: as a :  a disease, defect, or condition of the mind that renders one unable to understand the nature of a criminal act or the fact that it is wrong or to conform one's conduct to the requirements of the law being violated b :  inability to understand and participate in legal proceedings brought against one :  incompetence c :  inability to understand the nature and purpose of a punishment (as the death penalty) to which one has been sentenced d :  inability to understand the nature and consequences of one's acts (as making a will) or of events, matters, or proceedings in which one is involved — see also commitment, durham rule, irresistible impulse test, m'naghten test, not guilty by reason of insanity, substantial capacity test — compare capacity, competency, competent, diminished capacity, sanity

2 :  the affirmative defense of having acted while insane