Whilst there are many definitions of hypnosis available from which to choose, a common thread runs through all of them: hypnosis offers a method which allows changes in an individual's behaviour that has failed to respond to other approaches.
The hypnotist provides a stream of consciousness (simply by talking to the client or person being hypnotised) which replaces the stream of consciousness of the chattering mind of the client. If the hypnotist has the required skill and ability the client's mind happily accepts this stream of consciousness and is led by it into a state of deep relaxation where the conscious part of the mind becomes quiet and focussed elsewhere/nowhere. Self-consciousness is no longer apparent and the mind as a whole is completely open to the veracity of statements made to it concerning the nature of reality and its (the subject's mind) relationship to that reality. In accepting the truth of the suggestions made by the hypnotist, behavioural change in the client is thus made possible.
From a therapeutic perspective, hypnosis is an internally focused state of mind which is entered intentionally in order to achieve positive behavioural change. The hypnotic state can be achieved either alone through the process of self-hypnosis or through direction/guidance/suggestion by way of a hypnotist. In both cases an internal focus is achieved by the hypnotee wherein the subconscious mind is rendered open to inquiry and suggestion. It is this process which has been shown to be capable of bringing about lasting change in waking conscious experience and ongoing behaviour.
From a scientific perspective, the neural mechanism (that which is believed to be taking place in the brain during hypnosis) is currently a mystery which will remain so until more light is shed on the relationship between the brain and the mind. There is a further, perhaps unsolvable, problem here in that there is no scientific consensus of opinion as to precisely what the mind is or how it works. In our attempt to understand the relationship between the brain and consciousness we find that two very different disciplines are involved: psychology and neural science. Since both of these disciplines are constantly evolving there are currently many theories available from which to choose and these are being added to continuously.