An Etymological Dictionary of Astronomy and Astrophysics

An intervening substance through which an effect is produced; surrounding objects, conditions, or influences; environment. → interstellar medium.

Etymology (EN): From L. medium, from neut. of adj. medius “middle,” from PIE *medhyo- “middle;” cf. Av.
maδəma- “middle,” as below.

Etymology (PE): Madim, from Av. maδəma- [adj.] “middle, being in the middle; middling, of a middling size or quality,” maiδim “in the midst of,” maiδyāna- “the middle;” cf. Skt. mádhya- “middle;” Gk. messos; Goth. midjis; O.E. midd “middle;” O.C.S. medzu “between,” Arm. mej “middle;” L. medium, as above.