"pertaining to or situated in the middle, occupying a middle or intermediate position," 1590s, from French médian (15c.) and directly from Latin medianus "of the middle," from medius "in the middle" (from PIE root *medhyo- "middle"). Originally anatomical, of veins, arteries, nerves; general use is by 1640s. Median strip "narrow strip (paved or not) between lanes of a divided road" is by 1939, American English.
1540s, "a median part," originally anatomical, from Latin medianus "of the middle" (see median (adj.)). Statistical meaning "middle number of a series" is from 1883.