Webster's English Dictionary

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insanity

in.san.i.ty \in-'san-*t-e-\ n 1a: unsoundness or derangement of the mind 
   usu. occurring as a specific dis order (as schizophrenia) and usu. 
   excluding such states as mental deficiency, psychoneurosis, and various 
   character disorders 1b: a mental disorder  2: such unsoundness of mind or 
   lack of understanding as prevents one from h aving the mental capacity 
   required by law to enter into a particular relationship, status, or 
   transaction or as excuses one from criminal or civil responsibility 3a: 
   extreme folly or unreasonableness  3b: something utterly foolish or 
   unreasonable TIA denote serious mental disorder. INSANITY implies unfitness 
   to manage one's own affairs or safely enjoy liberty; LUNACY usu. implies 
   periodic disorder or alternating madness and lucidity; PSYCHOSIS is the 
   technical psychiatric term for a serious and prolonged behavioral disorder; 
   MANIA implies insanity but is often used specifically of one of the spells 
   of excitement that characterize some psychoses; DEMENTIA is a technical 
   psychiatric term that denotes mental deterioration whether psychogenic in 
   origin or resulting from damage to brain tissue SYN syn INSANITY, LUNACY, 
   PSYCHOSIS, MANIA, DEMEN