Magnetism -
See Animal Magnetism
Mania -
An energetic state of mental imbalance, characterized by high excitement.
Manic-depressive -
A mental disorder (psychosis) characterized by alternating states of high excitement followed by periods of depression. Now called bipolar depression.
Masochism -
Where pleasure is derived (usually sexual gratification) from the experience of pain or cruelty.
Mass Hypnosis -
Where a large group of people simultaneously experience a state of heightened suggestibility and become open to the experience of hypnotic phenomenon. 
Massed Practice -
A technique borrowed from behaviour therapy, where a patient is encouraged, either in or out of hypnosis, to purposefully repeat his symptom/s over and over. 
A typical use might be for a facial tick. The unconscious stimulus becomes exhausted (extinct) by the conscious repetition.
Megalomania -
Extreme self importance, self valuation. God complex.
Memory Manipulation -
Under hypnosis memories can be enhanced, removed or even changed. This facility of hypnosis is often used in therapy.
Mesmer, Franz Anton -
Born in Switzerland (1734 - 1815), became famous / notorious in Vienna and Paris by his making of cures using 'animal magnetism' ( the forerunner of today's hypnosis). Mesmer theorized that a subtle fluid permeated space and gathered in living things and this fluid (which was responsible for health) could be passed from one being to another, either directly or through the use of charged objects. He believed he used this fluid to magnetized patients and cure them. His theory was investigated by the scientific minds of the day (including Benjamin Franklin) and discredited. This caused the practice to fall from fashion. Fortunately many cures had been recorded and so investigation of his methods continued discretely in many countries. This research ultimately led to the understanding of the laws of suggestion and the acceptance of hypnosis as a natural mental state.
Mesmerism -
The type of 'hypnosis' that was practised by Mesmer and his followers. Typically theatrical and involving the use of 'hypnotic passes', where the hands are are moved along the shape of the body as if combing some invisible medium. Still practised today in eastern countries and in parts of Russia. Making something of a revival in some parts of America, mainly due to immigrants bringing these skills with them and the growing 'new age' belief in spiritual healing.
Metaphor -
From the word amphora, which is a vessel designed to carry or hold something, a  metaphor is likewise an image, phrase or story with an obvious meaning but which carries within it a secondary meaning. Metaphors are used widely in hypnotherapy to pass suggestions to the subconscious mind while bypassing or occupying critical faculties. Typically a short phrase or story that has more than one meaning and at least one of the inherent meanings carries a hypnotic suggestion. A hypnotic metaphor is like a Trojan Horse.
Migraine -
A severe headache, often accompanied by feelings of nausea. Often preceded by a warning 'aura'. Cases of recurring migraine can be treated most successfully with hypnosis. Caution must be exercised to ensure that the patient has had a thorough check up with a qualified medical practitioner first, in order to eliminate any possible organic origin.
Monoideism -
Describes a state of fixation on a single thought or topic. Encountered in hypnosis as concentration increases. Term coined by James Braid.
Mysophobia -
Excessive fear of dirt.
Mythomania -
Imaginary rationalization of acts and exaggerations on suggested themes, often encountered in deep hypnosis.