pl. of medium, on notion of "intermediate agency," a sense first found c.1600.
1580s, "a middle ground, quality, or degree," from L. medium, from neut. of adj. medius (see medial). Meaning "intermediate agency, channel of communication" is from c.1600.
That of "person who conveys spiritual messages" first recorded 1853, from notion of "substance through which something is conveyed."
Screen capture from: Le Feu Follet (1963) directed by Louis Malle