Insanity, madness, lunacy, and craziness are behaviors caused by certain abnormal mental or behavioral patterns. Insanity can manifest as violations of societal norms, including a person or persons becoming a danger to themselves or to other people. Conceptually, mental insanity also is associated with the biological phenomenon of contagion (that mental illness is infectious) as in the case of copycat suicides. In contemporary usage, the term insanity is an informal, un-scientific term denoting "mental instability"; thus, the term insanity defense is the legal definition of mental instability. In medicine, the general term psychosis is used to include the presence of delusions and/or hallucinations in a patient; and psychiatric illness is "psychopathology", not mental insanity.

In English, the word "sane" derives from the Latin adjective sanus, meaning "healthy". Juvenal's phrase mens sana in corpore sano is often translated to mean a "healthy mind in a healthy body". From this perspective, insanity can be considered as poor health of the mind, not necessarily of the brain as an organ (although that can affect mental health), but rather refers to defective function of mental processes such as reasoning. Another Latin phrase related to our current concept of sanity is compos mentis ("sound of mind"), and a euphemistic term for insanity is non compos mentis. In law, mens rea means having had criminal intent, or a guilty mind, when the act (actus reus) was committed.

A more informal use of the term insanity is to denote something or someone considered highly unique, passionate or extreme, including in a positive sense. The term may also be used as an attempt to discredit or criticize particular ideas, beliefs, principles, desires, personal feelings, attitudes, or their proponents, such as in politics and religion.

Madness, the non-legal word for insanity, has been recognized throughout history in every known society. Some traditional cultures have turned to witch doctors or shamans to apply magic, herbal mixtures, or folk medicine to rid deranged persons of evil spirits or bizarre behavior, for example. Archaeologists have unearthed skulls (at least 7000 years old) that have small, round holes bored in them using flint tools. It has been conjectured that the subjects may have been thought to have been possessed by spirits that the holes would allow to escape. More recent research on the historical practice of trepanning supports the hypothesis that this procedure was medical in nature and intended as means of treating cranial trauma.

The Greeks appeared to share something of the modern Western world's secular and holistic view, believing that afflictions of the mind did not differ from diseases of the body. Moreover, they saw mental and physical illness as a result of natural causes and an imbalance in bodily humors. Hippocrates frequently wrote that an excess of black bile resulted in irrational thinking and behavior.

Romans made other contributions to psychiatry, in particular a precursor of some contemporary practice. They put forward the idea that strong emotions could lead to bodily ailments, the basis of today's theory of psychosomatic illness. The Romans also supported humane treatment of the mentally ill, and in so doing, codified into law the principle of insanity as a mitigation of responsibility for criminal acts, although the criterion for insanity was sharply set as the defendant had to be found "non compos mentis", a term meaning "not sound of mind".

The Middle Ages typically continued a number of ideas of the Greeks and Romans, such as humoral theory, building on Galenic medicine, which remained foundational into the Modern Era. The Late Medieval and Early Modern period saw the rise of mentally impaired people employed as court jesters, which provided them a certain legal privilege. As in the classical world, certain bouts of madness could be associated with spiritual possession, and people who displayed psychoses could be seen in several ways depending on context, evoking anything from pity to revulsion, such as in the complex case of Charles VI of France, who variously suffered from memory loss, confusion, and glass delusion.^{[clarification needed]}

During the 18th century, the French and the British emphasised a need for humane treatment of the clinically insane, though the criteria for diagnosis and placement in an asylum were considerably looser than today, often including such conditions as speech disorder, speech impediments, epilepsy, and depression or being pregnant out of wedlock.