Английский Этимологический словарь - medium (n.)

- 1584, from L. medium, from neut. of adj. medius (see middle). Meaning "person who conveys spiritual messages" first recorded 1853, from notion of "substance through which something is conveyed." Artistic sense is from 1854. The adj. is 1670, from the noun. Happy medium is "golden mean," Horace's aurea mediocritas.