Although, hypnotists have been known and used since the beginning of mankind most experts have a hard time defining it. I feel most experts would agree on this definition: an altered state of consciousness where the subconscious level of the mind is in a state of hyper-suggestibility.
It is much easier to give examples of hypnosis than to define it. A most common example is one of driving down the road lost in thought or listening to some music and then realizing you just passed or almost passed your turn. Or, maybe you are driving somewhere and when you arrive, you don’t really remember the drive there or don’t remember passing some known landmarks. These are examples of light states of hypnosis, our subconscious mind was performing all the tasks necessary for us to drive the car while our conscious mind was somewhere else. Truck drivers call it, correctly so, “highway hypnosis.”
There are deeper states of hypnosis where body catalepsy can be achieved, analgesia or amnesia, even complete anesthesia can also be achieved. In the very deep states positive and negative hallucinations can be obtained. Positive hallucinations where the person will see and/or hear things which do not exist, or negative hallucinations where they will not see and or hear things that do exist. For therapy-type hypnosis, the lighter states are all that is required to achieve success for weight management, smoking cessation, stress reduction, etc.
For most stage hypnosis shows, the hypnotist generally would prefer working with people in the deeper states. The stage hypnotist may dismiss people from the stage if they feel the person is not moving into a deep enough state. This does not mean the person cannot be hypnotized, it just means the hypnotist feels the person will enjoy the show more as an audience member than a participant.