Yes, there is! Here's the medical and legal defition of insanity: "insanity n. 1) mental illness of such a severe nature that a person cannot distinguish fantasy from reality, cannot conduct her/his affairs due to psychosis, or is subject to uncontrollable impulsive behavior. Insanity is distinguished from low intelligence or mental deficiency due to age or injury. If a complaint is made to law enforcement, to the district attorney, or to medical personnel that a person is evidencing psychotic behavior, he/she may be confined to a medical facility long enough (typically 72 hours) to be examined by psychiatrists who submit written reports to the local superior/county/district court. A hearing is then held before a judge, with the person in question entitled to legal representation, to determine if she/he should be placed in an institution or special facility. The person may request a trial to determine sanity. The original hearings are often routine with the psychiatric findings accepted by the judge. In criminal cases, a plea of "not guilty by reason of insanity," will require a trial on the issue of the defendant's insanity (or sanity) at the time the crime was committed. In these cases the defendant usually claims "temporary insanity" (crazy then, but okay now). The traditional test of insanity in criminal cases is whether the accused knew "the difference between right and wrong," following the "M'Naughten Rule" from 19th Century England. Most states require more sophisticated tests based on psychiatric and/or psychological testimony evaluated by a jury of laypersons or a judge without psychiatric training. 4) a claim by a criminal defendant of his insanity at the time of trial requires a separate hearing to determine if a defendant is sufficiently sane to understand the nature of a trial and participate in his/her own defense. If found to be insane, the defendant will be ordered to a mental facility, and the trial held only if sanity returns." source: http://legal-dictionary.thefreedictionary.com/insanity

When the insanity defense was first admitted--I wish I could give you a time, date, and link--it applied to men who could "no more distinguish right from wrong than a brute beast." Since that time, the definition has been steadily expanded, because society doesn't like to face the reality that some people are simply no damn good. We prefer to think "They must be crazy." Well, most of them aren't. They know exactly what they are doing and do it with careful planning because they enjoy it. They aren't insane. They are wicked, vile, evil and fiendish.