Its easier to say what hypnosis is not than say precisely what it is. Opinions vary, but in my opinion, and from over twenty years experience, I would say that;
Ity isn't sleep, though there is such a thing as 'sleep hypnosis'.
It isn't relaxation, though many hypnotherapists use relaxation techniques.
It is not a state of amnesia ; you can be completely aware of what is happening and remember afterwards.
It does not require immobilisation ; if your nose itches, you can scratch it.
The list of what it isn't is endless. My own take on it is that hypnosis is simply a heightened state of communication between client and therapist: A state of rapport within which the clients subconscious mind is more easily able to find the resources required to 'move on'.