Healing by trance state has been acknowledged for centuries and has been observed, in one form or another,  in virtually every culture throughout the world.

However, the term "hypnosis" (from the Greek "hypnos" meaning "sleep") was

only coined around 1840 and remains a somewhat less than accurate description of the experience as, in most respects, the hypnotic state is entirely dissimilar to sleep.

At our current level of knowledge, the phenomenon cannot be conclusively defined but perhaps a reasonable interim definition might be that : "Hypnosis is a state of mind, enhanced by (although not exclusively) mental and physical relaxation,  in which our subconscious is able to communicate with our conscious mind"

This state of mind may be brought about either by oneself unaided (self-hypnosis) or with the help of another person. If this other person is a trained professional, who utilises the resultant state to encourage beneficial change to occur, the process is referred to as "Hypnotherapy"