Source : Webster's Revised Unabridged Dictionary (1913)

Submediant \Sub*me"di*ant\, n. (Mus.)
   The sixth tone of the scale; the under mediant, or third
   below the keynote; the superdominant.

Source : WordNet®

submediant
     n : (music) the sixth note of a major or minor scale (or the
         third below the tonic)