Source : Webster's Revised Unabridged Dictionary (1913)

Insanity \In*san"i*ty\, n. [L. insanitas unsoundness; cf.
   insania insanity, F. insanite.]
   1. The state of being insane; unsoundness or derangement of
      mind; madness; lunacy.

            All power of fancy overreason is a degree of
            insanity.                             --Johnson.

            Without grace The heart's insanity admits no cure.
                                                  --Cowper.

   2. (Law) Such a mental condition, as, either from the
      existence of delusions, or from incapacity to distinguish
      between right and wrong, with regard to any matter under
      action, does away with individual responsibility.

   Syn: Syn>- {Insanity}, {Lunacy}, {Madness}, {Derangement},
        {Aliention}, {Aberration}, {Mania}, {Delirium},
        {Frenzy}, {Monomania}, {Dementia}.

   Usage: Insanity is the generic term for all such diseases;
          lunacy has now an equal extent of meaning, though once
          used to denote periodical insanity; madness has the
          same extent, though originally referring to the rage
          created by the disease; derangement, alienation, are
          popular terms for insanity; delirium, mania, and
          frenzy denote excited states of the disease; dementia
          denotes the loss of mental power by this means;
          monomania is insanity upon a single subject.

Source : WordNet®

insanity
     n : relatively permanent disorder of the mind [ant: {sanity}]