Mediums – Shamans are sometimes distinguished from “mediums,” a term most often applied to people, generally in Spiritualist or Spiritist movements, who enter trances and possession states and allow the dead or “spirits” to speak through them to clients. The most common reasons for making this distinction are that mediums appear more passive than shamans, may be controlled by the spirits rather than masters of the spirits, and do not journey to other worlds but instead expect to be visited by the spirits. Since these features are certainly not absent among shamans, the distinction cannot be absolute. More tellingly, both trance and possession—indeed, all putative altered states of consciousness—are culturally mediated and interpreted performances and may neither be completely distinguished nor held to be entirely passive.
SOURCE:
Historical Dictionary of Shamanism by Graham Harvey and Robert J. Wallis 2007