Insanity, n.
Etymology: [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. - Insanity, Lunacy, Madness, Derangement, Aliention, Aberration, Mania, Delirium, Frenzy, Monomania, Dementia. 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.
Shades of meaning
Of these terms insanity is the most exact and comprehensive, including in its widest sense all morbid conditions of mind due to diseased action of the brain or nervous system, but in its more frequent restricted use applied to those forms in which the mental disorder is persistent, as distinguished from those in which it is temporary or transient. Craziness is a vague popular term for any sort of disordered mental action, or for conduct suggesting it. Lunacy originally denoted intermittent insanity, supposed to be dependent on the changes of the moon (L. luna): the term is now applied in general and legal use to any form of mental unsoundness except idiocy. Madness is the old popular term, now less common, for insanity in its widest sense, but with suggestion of excitement, akin to mania. In the derived sense, lunacy denotes what is insanely foolish, madness what is insanely desperate. Derangement is a common euphemism for insanity. Delirium is always temporary, and is specifically the insanity of disease, as in acute fevers. Dementia is a general weakening of the mental powers: the word is specifically applied to senile insanity, dotage. Aberration is eccentricity of mental action due to an abnormal state of the perceptive faculties, and is manifested by error in perceptions and rambling thought. Hallucination is the apparent perception of that which does not exist or is not present to the senses, as the seeing of specters or of reptiles in delirium tremens. Monomania is mental derangement as to one subject or object. Frenzy and mania are forms of raving and furious insanity. Compare synonyms for delusion; idiocy.