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1580s, "a middle ground, quality, or degree," from Latin medium "the middle, midst, center; interval," noun use of neuter of adjective medius (see medial (adj.)). 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." Artistic sense (oil, watercolors, etc.) is from 1854. Happy medium is the "golden mean," Horace's aurea mediocritas.
1660s, "average," from medium (n.). The Latin adjective was medius. Meaning "intermediate" is from 1796. As a size designation from 1711. as a designation of cooked meat, it is attested from 1931, short for medium-rare (1881).
medium me·di·um (mē'dē-əm)
n. pl. me·di·ums or me·di·a (-dē-ə)
Something, such as an intermediate course of action, that occupies a position or represents a condition midway between extremes.
An intervening substance through which something else is transmitted or carried on.
An agency by which something is accomplished, conveyed, or transferred.
The substance, often nutritive, in which a specific organism lives and thrives.
A culture medium.
A filtering substance, such as filter paper.