mediant (n.)

in music, "third note of the diatonic scale" (the one which determines whether the scale is major or minor), 1753, from Italian mediante, from Late Latin mediantem (nominative medians) "dividing in the middle," present participle of mediare "to be in the middle," from Latin medius "middle" (from PIE root *medhyo- "middle"). So called from being midway between the tonic and the dominant.