There are SO many different definitions of hypnosis, so I'm going to make it simple for you.
Hypnosis is an altered state of consciousness, in which a person is highly responsive to suggestion.
We actually go into hypnosis every day, without even realising it!
Here's one example of hypnosis that you may have experienced: Have you ever been driving along, and suddenly wonder how you got to your destination? The time seemed to go by, in a daze... and even though you may not recall the whole journey, you were always in control of the wheel.