Definition

The earliest definition of hypnosis was given by Braid, who coined the term "hypnotism" as an abbreviation for "neuro-hypnotism", or nervous sleep, which he opposed to normal sleep, and defined as: "a peculiar condition of the nervous system, induced by a fixed and abstracted attention of the mental and visual eye, on one object, not of an exciting nature."
Braid elaborated upon this brief definition in a later work:

[...] the real origin and essence of the hypnotic condition, is the induction of a habit of abstraction or mental concentration, in which, as in reverie or spontaneous abstraction, the powers of the mind are so much engrossed with a single idea or train of thought, as, for the nonce, to render the individual unconscious of, or indifferently conscious to, all other ideas, impressions, or trains of thought. The hypnotic sleep, therefore, is the very antithesis or opposite mental and physical condition to that which precedes and accompanies common sleep [...]

—Braid, Hypnotic Therapeutics, 1853

Therefore, Braid defined hypnotism as a state of mental concentration that often leads to a form of progressive relaxation, termed "nervous sleep". Later, in his The Physiology of Fascination (1855), Braid conceded that his original terminology was misleading, and argued that the term "hypnotism" or "nervous sleep" should be reserved for the minority (10%) of subjects who exhibit, substituting the term "monoideism", meaning concentration upon a single idea, as a description for the more alert state experienced by the others.
A new definition of hypnosis, derived from academic psychology, was provided in 2005, when the Society for Psychological Hypnosis, Division 30 of the(APA), published the following formal definition: