Médiumnité. Usually, a peculiarly passive state or condition of a person, due to "disease or to the exuberance of nervous fluid," either of which disturbs the normal balance of forces in his or her constitution. Thereby, the man or woman, becoming unconscious at times of his natural senses, is then made the automatic agent of various psycho-astral forces and entities, and these last are of several kinds: elementaries, astral shells or spooks, nature sprites, and astral and even physical elementals.
This entranced state is cultivated in modern spiritualism as a means of inviting spirit-control and of gaining special knowledge. However, the very relation of the seven human principles infallibly and necessarily prevents pure spirit from directly contacting physical matter. In the complete living man on earth, his spiritual nature -- buddhi -- is above, within, or beyond his higher mind (higher manas) yet can only act downwards through it. The spiritual does not directly contact or act through the lower mind and emotions (kama-manas). After death, the higher triad (atma-buddhi-manas) separates from the lower quaternary and ascends to its own realms, entirely beyond the reach of the personal man that was. Mediumship, moreover, is a negation of conscious selfhood and a reversal of natural evolutionary growth, whereby the reincarnating ego involved in material existence comes forth, step by step, taking positive, conscious control of its body, mind, and emotions. Our racial evolution reached the depths of materiality in Atlantean times, and therefrom made the turn onto the ascending arc. Hence, our future progress consists, not in trying further to materialize spirit, but in progressively spiritualizing matter.
Human mediumship is a voluntary, or more often involuntary, subjection to the lower planes of astral substance which, while more ethereal than ordinary matter, yet are of a quality more gross, more powerful, and usually more malefic. Entrance into these astral realms produces a species of astral intoxication, from the delusion of strange because unknown and often unequilibrated forces, deceptive astral pictures; and the astral intoxication is increased because of considering these experiences as wonder-phenomena. In other words, the conditions and experiences sensed are as genuine, and as unreliable and utterly useless, as are the hallucinations of the delirious or insane. Only an occultist of masterful will and great purity of life can rise consciously to the spiritual plane and, looking down on the astral levels below, understand, control, and remember what he sees. In untrained mediumship the atoms and molecules of the astrally "controlled" body which the alien astral entity uses to mold into a form and to move with its own desire-impulses, retain this astral psychomagnetic imprint. With repeated trances, the medium grows continuously and progressively less than his individual self, because of his thoughts and feelings becoming mixed with, overlaid, or blurred by ideas and emotions which per se are abnormal and misleading. He therefore becomes irresponsible as a source of genuine spiritual knowledge and prevision, and still less responsible as a guardian of sacred truths. Because of this, untrained mediumship precludes initiation into the Mysteries as the person's faith in his astral "control" would dominate him instead of the rules of the sanctuary.
(source : "Encyclopedic Theosophical Glossary", traduction d'Esopedia)