The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            All power of fancy over reason is a degree of
            insanity.                             --Johnson.
      [1913 Webster]

            Without grace
            The heart's insanity admits no cure.  --Cowper.
      [1913 Webster]

   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: Insanity, Lunacy, Madness, Derangement,
        Alienation, 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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

insanity
    n 1: relatively permanent disorder of the mind [ant: saneness,
         sanity]

Moby Thesaurus II by Grady Ward, 1.0:

116 Moby Thesaurus words for "insanity":
   aberration, absurdity, alienation, amentia, arrested development,
   asininity, backwardness, battiness, blithering idiocy,
   brain disease, brainlessness, buffoonery, clownishness, crack-up,
   crackpottedness, crankiness, craziness, cretinism, daffiness,
   delirium, delusion, dementia praecox, derangement, desipience,
   distraction, dotage, dottiness, eccentricity, emotional disorder,
   emotional instability, fatuity, fatuousness, folly, foolery,
   foolheadedness, foolishness, frenzy, frivolity, frivolousness,
   functional nervous disorder, giddiness, goofiness, half-wittedness,
   hallucination, hysteria, idiocy, idiotism, illusion, imbecility,
   impracticality, inanity, ineptitude, infantilism, insaneness,
   irrationality, irresponsibility, lunacy, madness, maladjustment,
   mania, manic-depressive psychosis, melancholia,
   mental defectiveness, mental deficiency, mental derangement,
   mental disorder, mental handicap, mental illness,
   mental retardation, mindlessness, mongolianism, mongolism,
   mongoloid idiocy, moronism, moronity, nervous breakdown,
   nervous disorder, neurosis, niaiserie, nonsense, nugacity,
   nuttiness, paranoia, personality disorder, preposterousness,
   problems in living, profound idiocy, psychoneurosis, psychopathy,
   psychosis, queerness, reaction, retardation, retardment, sappiness,
   schizophrenia, screwiness, senselessness, silliness,
   simple-wittedness, simplemindedness, simpleness, simplicity,
   social maladjustment, stupidity, subnormality, thoughtlessness,
   triflingness, triviality, unbalance, unreasonableness, wackiness,
   weirdness, witlessness, zaniness, zanyism



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INSANITY, med. jur. A continued impetuosity of thought, which, for the time
being, totally unfits a man for judging and acting in relation to the matter
in question, with the composure requisite for the maintenance of the social
relations of life. Various other definitions of this state have been given,
but perhaps the subject is not susceptible of any satisfactory definition,
which shall, with, precision, include all cases of insanity, and exclude all
others. Ray, Med. Jur. Sec. 24, p. 50.
     2. It may be considered in a threefold point of view: 1. A chronic
disease, manifested by deviations from the healthy and natural state of the
mind, such deviations consisting in a morbid perversion of the feelings,
affections and habits. 2. Disturbances of the intellectual faculties, under
the influence of which the understanding becomes susceptible of
hallucinations or erroneous. impressions of a particular kind. 3. A state of
mental incoherence or constant hurry and confusion of thought. Cyclo.
Practical Medicine, h. t.; Brewster's Encyclopaedia, h. t.; Observations on
the Deranged Manifestations of the Mind, or Insanity, 71, 72; Merl. R‚pert.
mots Demenoe, Folie, Imbecilite; 6 Watts & Serg. 451.
     3. The diseases included under the name of insanity have been arranged
under two divisions, founded on two very different conditions of the brain.
Ray, Med. Jur. ch. 1, Sec. 33.
     4.-1. The want of, or a defective development of the faculties. 1st.
Idiocy, resulting from, 1. Congenital defect. 2. An obstacle to the
development of the faculties, supervening in infancy. 2d. Imbecility,
resulting from, 1. Congenital defects. 2. An obstacle to the development of
the faculties, supervening in infancy.
     5.-2. The lesion of the faculties subsequent to their development. In
this division may be classed, 1st. Mania, which is, 1. Intellectual, and is
general or partial. 2. Affective and is general or, partial. 2d. Dementia,
which is, 1. Consecutive to mania, or injuries of the brain. 2. Senile, or
peculiar to old age.
     6.-There is also a disease which has acquired the name of Moral
insanity. (q. v.)
     7. Insanity is an excuse for the commission of acts which in others
would be crimes, because the insane man has no intention; it deprives a man
also from entering into any valid contract. Vide Lunacy; Non compos mentis,
and Stock on the Law of Non Compotes Mentis; 1 Hagg. Cons. R. 417; 3 Addams,
R. 90, 91, 180, 181; 3 Hagg. Eccl. R. 545, 598, 600; 2 Greenl. Ev. Sec. 369,
374; Bouv. Inst. Index, h. t.