in·san·i·ty (n-sn-t)
n. pl. in·san·i·ties
1. Mental illness or derangement. No longer in scientific use.
2. Law
a. Unsoundness of mind sufficient in the judgment of a civil court to render a person unfit to maintain a contractual or other legal relationship or to warrant commitment to a mental health facility.
b. In most criminal jurisdictions, a degree of mental malfunctioning sufficient to relieve the accused of legal responsibility for the act committed.
3.
a. Extreme foolishness; folly.
b. Something that is extremely foolish.
Synonyms: insanity, lunacy, madness, mania, dementia
These nouns denote conditions of serious mental disability. Insanity is a grave, often prolonged condition that prevents a person from being held legally responsible for his or her actions: was judged not guilty for reasons of insanity.
Lunacy often denotes derangement relieved intermittently by periods of clear-mindedness: yelled wildly in a moment of utter lunacy.
Madness often stresses the violent aspect of mental illness: a story about obsession and madness.
Mania refers principally to the excited, or manic, phase of bipolar disorder: prescribed drugs to control the patient's periods of mania.
Dementia implies mental deterioration brought on by an organic brain disorder: underwent progressive stages of dementia.

