These words refer to serious disturbances of the mind. Psychotic is the most precise of these, being used by psychiatrists to describe someone who exhibits a break with or withdrawal from reality; the word contrasts with neurotic, which describes someone with a distorted view of reality. A psychotic personality may be anti-social, violent or merely passive: psychotic delusions; psychotic acts of cruelty; psychotic apathy. The word may also be used by sociologists to describe, less precisely, acts that are extreme departures from social norms: a psychotic split between duty and simply humanity that resulted in the setting-up of the concentration camps. The word has also entered the popular consciousness, where it can be used imprecisely as a simply term of opprobrium: my psychotic mother-in-law.

Psychopathic is even more likely to be used in a popular context, although here it is more often a precise high-toned substitute for vicious or brutal: a psychopathic killer. In its strict psychiatric use, however, it refers to someone who exhibits no break with reality, unlike the psychotic , but who has no sense of social responsibility; while this may result in violent or criminal acts, it may also result in extreme withdrawal or passivity.

Demented at its most precise indicates someone whose mentality has degenerated from a previous level that was more nearly normal: brain damage that left him demented . More often the word is used loosely to refer to anyone who seems possessed by odd obsessions or eccentricities: her demented fear of strangers. Insane , a word now abandoned by most psychologists and psychiatrists, it still used legally to refer to someone who is so mentally disturbed as to be unable to distinguish morally between right and wrong. In popular usage, it may refer also to things that appear foolish or ridiculous: a trial that hinged on whether or not the accused could be considered insane at the time of the murder; an insane scheme for making a million on the stock exchange. The word can even be used as a superlative for anything intense, exciting or fun: an absolutely insane beach party. Deranged is less likely to be used in either a psychological or legal context, but, in a general way, it has validity in formal usage to refer to mental or emotional drives or balances that have become disordered: deranged by fever; deranged from a lack of food and water; She suffered a sudden shock that deranged her completely.

The remaining words no longer have valid uses in either psychological or legal discussion. Lunatic , in fact, is now less often used on any level of speech because of its old-fashioned sound and its overtones of ignorance and superstition; its root refers to a belief that the moon could cause mental disturbance. Sometimes it is still used comically or disapprovingly for odd or undesirable behaviour: a lunatic notion that anyone should be able to own lethal weapons. It is also commonly found in the expression "lunatic fringe," which denotes those followers or devotees of a movement, idea, etc., who are extreme or fanatical in their enthusiasm. By contrast, mad and crazy are used widely to refer popularly to mental disturbance. Mad , the more formal of the two, may suggest profound mental disorder. More informally, the word can also refer to anything silly, flamboyant pointless, unrealistic or irrational: a mad feathered hat; a mad hope that help might still come; a mad desire to wreak vengeance on his imagined oppressor. Careful writers avoid the word when it means angry: remarks that made her mad at me. Crazy is the most informal of these and refers popularly to someone who is extremely neurotic or psychotic. The word also has a wide range of additional uses to refer to the eccentric, odd, troubled or desiring: a crazy collection of furniture; a crazy dread of failing in the exam; crazy about sailors. It can even express approval for something that goes to extremes: a crazy New Year’s Eve party.