There are very many definitions of Hypnosis available.
As a Hypnotist, I would say that most simply, Hypnosis is a way to relax your body and your mind to allow access and communication with your subconscious mind. Then as a trained and certified Hypnotist and Hypnotherapist, I work with you to achieve specific goals and outcomes by communicating to your subconscious mind carefully worded and crafted directions and requests.
Here is another definition from the internet.
Hypnosis is commonly viewed as a natural, altered state of consciousness, where the conscious (analytical) mind is bypassed, and the subconscious (creative) mind is accessed. This allows the client to use the power of visualization and suggestion, given by oneself or another (the Hypnotist or Hypnotherapist), to change and improve behaviour patterns.
Hypnosis, in itself, is just a state of mental and physical relaxation, along with a more focused sense of concentration. Hypnosis is not sleep (as is often popularly assumed), and most people find that they are more aware of smells, sounds, and feelings than usual. This concentrated awareness is what allows the Hypnotherapist to plant positive suggestions and images in the mind of the client to bring about lasting changes.
Focused attention
This school of thought holds that hypnosis as a state is very similar to other states of extreme concentration, where a person becomes oblivious to his or her surroundings while lost in thought. Often suggested as an example is when a driver suddenly finds oneself much further down the road without any memory of driving the intervening distance, or when a person is watching television and focuses so intently on the program that he or she ceases to be aware of the sides of the screen.
The act of hypnotizing, is, in effect, the act of manually inducing a similar state.