The Collaborative International Dictionary of English v.0.48:

Subdominant \Sub*dom"i*nant\, n. (Mus.)
   The fourth tone above, or fifth below, the tonic; -- so
   called as being under the dominant.
   [1913 Webster]

WordNet (r) 3.0 (2006):

subdominant
    n 1: (music) the fourth note of the diatonic scale