Definition of: insanity

(in·san′ə·tē) noun
1. Any mental disorder characterized by temporary or permanent irrational or violent deviations from normal thinking, feeling, and behavior: not a technical term in medicine or psychiatry.
2. Law Any degree of mental unsoundness resulting in inability to distinguish between right and wrong, to control the will, foresee the consequences of an act, make a valid contract, or manage one's own affairs.
3. Lack of sound sense; extreme folly. Synonyms: alienation, craziness, delirium, dementia, derangement, frenzy, lunacy, madness, mania, monomania. Of these terms insanity is the most comprehensive, including in a loose sense all morbid conditions of mind due to diseased action of the brain or nervous system. 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). Madness is the old popular term, now suggesting excitement akin to mania. 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 the mental incapacities of senility. Monomania is mental derangement as to one subject or object. Frenzy and mania are forms of raving and furious insanity. Compare DELUSION, FRENZY, IDIOCY. Antonyms: clearness, lucidity, rationality, sanity.