A substance through which an effect is transmitted from one
   thing to another; as, air is the common medium of sound. Hence: The
   condition upon which any event or action occurs; necessary means of
   motion or action; that through or by which anything is accomplished,
   conveyed, or carried on; specifically, in animal magnetism,
   spiritualism, etc., a person through whom the action of another being
   is said to be manifested and transmitted.