Etymology
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medium (n.)

1580s, "a middle ground, quality, or degree; that which holds a middle place or position," from Latin medium "the middle, midst, center; interval," noun use of neuter of adjective medius "in the middle, between; from the middle" (from PIE root *medhyo- "middle").

Many of the secondary senses are via the notion of "intervening substance through which a force or quality is conveyed" (1590s) and "intermediate agency, channel of communication" (c. 1600). From the former, via application to air, etc., comes the sense of "one's environment or conditions" (1865). From the latter comes the sense of "a print publication" (1795) which later grew into the meaning in media.

In spiritualism, "person who conveys spiritual messages," by 1853. In painting, in reference to oil, watercolor, etc., by 1854. The notion is "liquid with which pigments are ground or mixed to give them desired fluidity." Happy medium is the "golden mean," Horace's aurea mediocritas.

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medium (adj.)

1660s, "average, middling," from medium (n.). The Latin adjective was medius. Meaning "intermediate" is from 1796. As a designation of size or weight, by 1711. As a designation of cooked meat between well-done and rare, it is attested from 1931; earlier was medium-rare (1881).

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media (n.)

"newspapers, radio, TV, etc." 1927, perhaps abstracted from mass-media (1923, a technical term in advertising); plural of medium (n.) as "intermediate agency," a sense attested in English from c. 1600. Also see -a (2).

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*medhyo- 
Proto-Indo-European root meaning "middle." Perhaps related to PIE root *me- (2) "to measure."

It forms all or part of: amid; intermediate; mean (adj.2) "occupying a middle or intermediate place;" medal; medial; median; mediate; medieval; mediocre; Mediterranean; medium; meridian; mesic; mesial; meso-; meson; Mesopotamia; Mesozoic; mezzanine; mezzo; mezzotint; mid (prep., adj.); middle; Midgard; midriff; midst; midwife; milieu; minge; mizzen; moiety; mullion.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit madhyah, Avestan madiya- "middle," Greek mesos, Latin medius "in the middle, between; from the middle," Gothic midjis, Old English midd "middle," Old Church Slavonic medzu "between," Armenian mej "middle."
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milieu (n.)

"surroundings, medium, environment," 1854, from French milieu, "middle, medium, mean," literally "middle place" (12c.), from mi "middle" (from Latin medius, from PIE root *medhyo- "middle") + lieu "place" (see lieu).

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pollutant (n.)

polluting agent or medium," "1888, from pollute + -ant. Related: Pollutants.

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psychic (n.)

"a medium, person susceptible to psychic impressions," 1870; see psychic (adj.).

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middling (adj.)

"medium in rank, condition, or degree; intermediate," 1540s, from Middle English medlinge "intermediate between two things" (late 14c.), from middle (adj.) + present-participle suffix -ing (2). Used in trade to designate the second of three grades of goods. Hence "only medium, neither good nor bad" (1650s). As an adverb, "tolerably, passably," by 1719.

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brain-wave (n.)
"apparent telepathic vibration transferring a thought from one person to another without any other medium, 1869, from brain (n.) + wave (n.).
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mass-media (n.)

singular mass-medium, "means of communication that reach large numbers of people," 1923; see mass (n.1) + media (n.).

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