Hypnosis is a human condition involving focused attention, reduced peripheral awareness, and an enhanced capacity to respond to suggestion.
There are competing theories explaining hypnosis and related phenomena. Altered state theories see hypnosis as an altered state of mind or trance, marked by a level of awareness different from the ordinary state of consciousness. In contrast, nonstate theories see hypnosis as, variously, a type of placebo effect, a redefinition of an interaction with a therapist, or form of imaginative role enactment.

Source : Wikipedia

The history of human hypnosis (or similar phenomena) goes back many hundreds, probably thousands of years. There isn't any doubt that hypnotic states exist, can be physiologically measured, and can be used in various ways, therapeutic and otherwise.

There is currently no agreed physiological explanation as to how it comes about.

Note: Some animals have been found to be hypnotisable, e.g chickens, fish, etc. ^{[ref. needed]}

Also see : Suggestibilityplugin-autotooltip__plain plugin-autotooltip_bigSuggestibility

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Suggestibility can be roughly defined as 'the quality of being inclined to accept and act on the suggestions of others' - and is an essential component of Hypnosis. That's to say, the more 'suggestible' an individual is, the more easily she/he can be hypnotised.