The Collaborative International Dictionary of English v.0.48:

Subtonic \Sub*ton"ic\, a. (Phonetics)
   Applied to, or distinguishing, a speech element consisting of
   tone, or proper vocal sound, not pure as in the vowels, but
   dimmed and otherwise modified by some kind of obstruction in
   the oral or the nasal passage, and in some cases with a
   mixture of breath sound; -- a term introduced by Dr. James
   Rush in 1833. See Guide to Pronunciation, [sect][sect]155,
   199-202.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Subtonic \Sub*ton"ic\, n.
   1. (Phonetics) A subtonic sound or element; a vocal
      consonant, as b, d, g, n, etc.; a subvocal.
      [1913 Webster]

   2. (Mus.) The seventh tone of the scale, or that immediately
      below the tonic; -- called also subsemitone.
      [1913 Webster]

WordNet (r) 3.0 (2006):

subtonic
    n 1: (music) the seventh note of the diatonic scale [syn:
         subtonic, leading tone]