1560s, "pertaining to a mathematical mean," from Late Latin medialis "of the middle," from Latin medius "in the middle," from PIE *medhyo- "middle" (cognates: Sanskrit madhyah, Avestan madiya- "middle," Greek mesos, Gothic midjis, Old English midd "middle," Old Church Slavonic medzu "between," Armenian mej "middle"); perhaps related to PIE root *me- "between." Meaning "occupying a middle position" is attested from 1721.

"a medial letter," 1776, from medial, adj..