Hypnosis is an altered state of awareness where the person in TRANCE tends to get absorbed in the object of awareness with relative suppression of peripheral awareness.

There is a change in the brain wave activity similar to that moment after which one drifts into deep sleep. This state just before drifting into deep sleep is called ALPHA STATE.

Brain's waking state is BETA, just as you are going to sleep it changes to ALPHA, then to THETA and onward to DELTA, the state of DEEP SLEEP. 

The ALPHA STATE is the TRANCE STATE and is a very dreamy & pleasant state in which the mind is very open to suggestions/ visualisation and creating a rich sensory experience.

Hypnotherapy is therapy provided under trance state by way of giving necessary affirmations and suggestions for bringing about desired behavioral changes in the patient.