1869, "the coming on of sleep," coined (as an alternative to hypnotism) from Greek hypnos "sleep" (see somnolence) + -osis "condition." Of an artificially induced condition, from 1880.
hypnosis hyp·no·sis (hĭp-nō'sĭs)
n. pl. hyp·no·ses (-sēz)
A trancelike state resembling somnambulism, usually induced by another person, in which the subject may experience forgotten or suppressed memories, hallucinations, and heightened suggestibility.
A sleeplike state or condition.
Hypnotism.
Placing persons in a drowsy, sleeplike state in which they allegedly become vulnerable to the suggestions made by the hypnotist. Hypnosis may also be used to tap into the unconscious and is often characterized by vivid recall of memories and fantasies. These properties make hypnosis a useful tool in psychotherapy. Hypnosis also has sinister implications, for subjects may be manipulated to perform embarrassing actions or be susceptible to carrying out the hypnotist's commands after the hypnosis session (posthypnotic suggestion).