This article describes the history, rationale, and guidelines for developing a new definition of hypnosis by the Society of Psychological Hypnosis, Division 30 of the American Psychological Association. The definition was developed with the aim of being concise, heuristic, and allowing for alternative theories of the mechanisms (to be determined in empirical scientific study). The definition of hypnosis is presented as well as definitions of the following related terms: hypnotic induction, hypnotizability, and hypnotherapy. The implications for advancing research and practice are discussed. The definitions are presented within the article.