By strict definition, "somnambulism" means "sleepwalking" and a "somnambulist" is someone who sleepwalks. But these terms have also been adapted for use in the hypnosis community. Until you have experienced hypnosis for yourself, often the closest thing you have to compare it to is that fuzzy state that is somwhere in between being awake and being asleep referred to as a "hypnogogic" state. Because of that, hypnotists often use the word "sleep" in conjunction with their work and terms related to sleep are generally easier for someone who does not have first hand knowlege of the effects of hypnosis to understand.
In the context of hypnosis, somnambulism can be defined this way:
A deep state of hypnosis, where the client may respond with amnesia, anesthesia, negative and positive hallucinations, and complete control of the senses. It is characterized by the eyes rolling up underneath the eyelids. Somnambulism IS NOT "sleepwalking" as sleepwalking often occurs without any conscious awareness. In the hypnosis context, "somnambulism" refers to the ability to transition smoothly and easily between the conscious and unconscious experience.
A person capable of reaching a profound state of hypnotic trance or somnambulism. Somnambulism can be "learned" with practice and experience. Somnambulists are able to quickly and easily slip in and out of various altered mental states that we recognize as "hypnosis".
A somnambulist who is capable of reaching that level of hypnotic trance known and somnambulism without training or practice, generally during their first experience with hypnosis. In Kappasinian terms, this is most likely with a person who responds equally well to all suggestions, both direct and indirect, affecting both the body and emotions. This person would have a near 50/50 Suggestibility (50% Emotional Suggestible and 50% Physical Suggestible).
Want to know if you are a natural somnambulist? Take the test here: Hypnotechs Kappasinian E&P Suggestibility Assessment
Or learn more about "levels" of hypnosis with: Arons Scale of Hypnotic Depth