Médium. Anything that serves as an intermediate, especially applied by modern spiritualists to a person who, alleged to be under the "control" of some other being, usually invisible, becomes a transmitting medium for phenomenal messages, feelings, or actions. These entities, mistakenly called spirits of the dead, are no part of the spiritual nature of composite man. On the contrary, these communications come from various entities in the astral world which interpenetrates and surrounds the physical earth, just as our astral model-body and aura surround the interpenetrate our physical form, cell for cell. In our present state of evolution, the astral or model-body acts normally only when conjoined to the physical -- a natural provision for protection from conditions with which we are as yet evolutionally unprepared to deal. The medium, however, is one who is born with or develops a peculiarly unstable and often actually dislocated state of the elements of his inner constitution. Thereby he becomes at times disorganized physiologically and in his nervous system, which connects the inner man with the outer world, and he suffers, in effect, a psychic dislocation. Then the entranced, unconscious medium functions with magnetic sympathy with currents and entities in the astral light, especially with those in the kama-lokic levels which are nearest the earth. Of these many entities, the types usually manifesting are nature spirits or elements of various kinds; kamic remnants, the shells or spooks of the dead; and elementaries or the imperfect astral remains of excarnate human beings who when alive on earth showed marked tendencies to gross and evil living. Being fated, because of their strongly materialistic biases and appetites, to exist in the astral realm, these last are a peculiarly dangerous and demoralizing influence, especially to people of weak will or of mediumistic temperament. Without physical body or real conscience, the elementaries yet are living entities of the unexpended force of their earth-passions and desires, eager to occupy and use a living body, meantime absorbing its vital essence if they can make psychic contact with it. They are psychomagnetically drawn to such conditions as the seance room usually offers. The delicate tingling on the medium's skin, supposed to come from angelic fingers, is actually an astral emanation of vitality to form an atmosphere or aura for the besieging control. These feathery touches are like the aurae which often precede convulsive epileptic attacks where the pale, cold, unconscious body of the ousted sufferer becomes temporarily possessed. Each time when the passive medium is controlled, his spiritual will is progressively weakened, his higher mind is blurred, and he becomes an open door for all kinds of uncanny astral influences. It is true that psychic sensitives of clean life and honest purpose, may first attract entities belonging to higher kama-lokic levels. But the finest types of supposed spirit faces that they see are generally reflections from their own mental pictures of beloved ones, or of their own innate ideals.
Of the many types of astral elementals, connection with even those friendly to man are injurious, for they all use part of the living for their automatic actions. Moreover, black magicians who live in their kama-rupas -- in the astral world -- relatively few though they are, survive by using many of these nature spirits to vampirize vitality from the living. The elementaries who, unfortunately, are galvanized into a fictitious life by devitalizing the medium and the sitters -- as clairvoyants have often seen -- are making new evil karma, and even inviting final spiritual disaster.
At most, this dealing with the dead is necromancy -- a wrong condemned by the wise in all ages as misleading at the least, very dangerous and ethically demoralizing at the worst. The passive medium under alien astral control, is the very antithesis of the highly evolved human mediator whose awakened spiritual, intellectual, and psychic nature serves as a conscious channel of inspiring influence between lofty spiritual powers and ordinary men, or between mahatmas of the Great White Lodge and men.
(source : "Encyclopedic Theosophical Glossary", traduction d'Esopedia)