\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.