A special psychological state with certain physiological attributes, resembling sleep only superficially and marked by a functioning of the individual at a level of awareness other than the ordinary conscious state. This state is characterized by a degree of increased receptiveness and responsiveness in which inner experiential perceptions are given as much significance as is generally given only to external reality.

The hypnotized individual appears to heed only the communications of the hypnotist. He seems to respond in an uncritical, automatic fashion, ignoring all aspects of the environment other than those pointed out to him by the hypnotist. He sees, feels, smells, and otherwise perceives in accordance with the hypnotist's suggestions, even though these suggestions may be in apparent contradiction to the stimuli that impinge upon him. Even the subject's memory and awareness of self may be altered by suggestion, and the effects of the suggestions may be extended (post-hypnotically) into the subject's subsequent waking activity.

Hypnosis has been officially endorsed as a therapeutic method by medical, psychiatric, dental, and psychological associations throughout the world. It has been found most useful in preparing people for anesthesia, enhancing the drug response, and reducing the required dosage. In childbirth it is particularly helpful, because it is effective in alleviating the mother's discomfort while avoiding drug-induced impairment of the child's physiological function. Hypnosis is highly regarded in the management of otherwise intractable pain, including that of terminal cancer. It is valuable in reducing the widespread fear of dental procedures; the very people whom dentists find most difficult to treat frequently respond best to hypnotic suggestion.

In the area of psychosomatic medicine, hypnosis has been used in a variety of ways. Patients have been trained to relax and to carry out, in the absence of the hypnotist, exercises that have had salutary effects on some forms of high blood pressure, headaches, and functional disorders.

In psychotherapy, hypnosis has been used in a variety of ways. For example, the technique of revivifying traumatic events in order to produce an emotional catharsis continues to be a useful treatment in relieving neuroses with traumatic onset, such as those that develop in combat, among individuals with relatively stable prior adjustments.
Though the induction of hypnosis requires little training and no particular skill, when used in the context of medical treatment, it should never be employed by individuals who do not have the competence and skill to treat such problems without the use of hypnosis.

Hypnosis has been repeatedly condemned by various medical associations when it is used purely for purposes of public entertainment, owing to the danger of persons suffering adverse posthypnotic reactions to the procedure.
Indeed, in this regard, several nations have banned or limited commercial or other public displays of hypnosis on such grounds.