Hypnosis is an artificially induced mental state characterized by an individual's loss of critical powers and consequent openness to suggestion. It may be induced by an external agency or by the individual herself (autohypnosis). It has been widely used in medicine (usually to induce analgesia) and especially in psychiatry and psychotherapy. Here, the particular value of hypnosis is that, while in trance, the individual may be encouraged to recall deeply repressed memories that may be the heart of, for example, a complex; once such causes have been elucidated, therapy may proceed.

Hypnosis goes back far in antiquity. However, the first definite information on it comes in the late 18th century with the work of Mesmer, who held that disease was the result of imbalance in the patient's "animal magnetism," and hence attempted to cure by the use of magnets. In some of his patients were cured, presumably by suggestion; and the term mesmerism is still sometimes used for hypnotism. Early psychotherapeutic uses include that of Jean-Martin Charcot and his pupil Sigmund Freud; though Freud later rejected hypnosis and used instead his own technique, free association. Little is known of the nature or cause of hypnotism, and its amateur use is potentially dangerous.