The simple definition of hypnosis is:
"The bypass of the Critical Faculty of the conscious mind, and the establishment of acceptable selective thinking."
It is not psychology. The practice of psychology is to spar with the critical faculty; to wrestle the conscious components of the mind until the desired results are attained.  Hypnosis simply bypasses the Critical Faculty (or Factor) without challenging any of the conscious components.
For a more detailed definition of hypnosis see Q & A
And, if you like to read, ckick here.