insanity

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Severe mental illness or derangement. Not used in psychiatric diagnosis.
• noun Unsoundness of mind sufficient to render a person unfit to maintain a contractual or other legal relationship or sufficient to warrant commitment to a mental health facility.
• noun Incapacity to form the criminal intent necessary for legal responsibility, as when a mental disorder prevents a person from knowing the difference between right and wrong.
• noun Incapacity because of a mental disorder to participate adequately as a defendant in criminal proceedings or to understand the imposition of a criminal punishment, particularly the death penalty.
• noun Extreme foolishness or irrationality.
• noun Something that is extremely foolish, distinctly irrational, or wildly excited.

from The Century Dictionary.

• noun A seriously impaired condition of the mental functions, involving the intellect, emotions, or will, or one or more of these faculties, exclusive of temporary states produced by and accompanying acute intoxications or acute febrile diseases.
• noun In its legal use, insanity consists in the lack of such mental soundness as renders a person criminally responsible, or capable of making a valid contract, conveyance, or will, or of managing his own affairs. There is great difference of opinion as to what extent of disease or imperfect development, and what, if any, aberrations of mind not traceable to disease or imperfection of the brain, should be regarded as constituting this degree of mental unsoundness. The tendency of legal opinion has long been to enlarge the scope of the word, and extend the rules as to insanity to derangements not recognized in earlier times, when insane was used as the equivalent of mad or lunatic, and insanity generally implied furor or mania, or, at least, obvious forms of total defect of responsible understanding. What constitutes legal insanity—that is, exonerative or incapacitating insanity—in doubtful cases is now universally regarded as depending upon the relation between the defect in the particular mind and the nature of the act in question. Thus, insanity, as a defense in criminal law, means, according to the rule in England and in many of the United States, incapacity to distinguish between right and wrong, in respect to the act in question, or incapacity to be conscious of acting contrary to law; while by some authorities inability to control the will (irresistible morbid impulse) also is recognized as insanity. Insanity in reference to the law of contracts is generally understood to mean such a defect as incapacitates from a rational assent, considering the nature of the contract, whether marriage, partnership, sale, etc. Insanity (or, as more commonly expressed, unsoundness of mind) in reference to the law of wills is generally understood to mean such a defect as incapacitates from knowing or collecting in mind the facts respecting the property to be disposed of and the persons naturally and justly to be considered in its disposal, or from making an intelligent and rational choice as to its disposal. It has often been defined too narrowly, as consisting only in delusion. It is a disputed question whether the existence of disease or defect in the brain itself is an essential fact. Some of those who hold that one or the other always characterizes insanity are understood to assert that the disease or defect may be inferred from the irrationality of conduct, without other independently adequate evidence.

from the GNU version of the Collaborative International Dictionary of English.

• noun The state of being insane; unsoundness or derangement of mind; madness; lunacy.
• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The state of being insane; madness.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun relatively permanent disorder of the mind