The Collaborative International Dictionary of English v.0.48:

Mediant \Me"di*ant\, n. [L. medians, p. p. of mediare to halve:
   cf. It. mediante, F. m['e]diante.] (Mus.)
   The third above the keynote; -- so called because it divides
   the interval between the tonic and dominant into two thirds.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mediant
    n 1: (music) the third note of a diatonic scale; midway between
         the tonic and the dominant