Source : Webster's Revised Unabridged Dictionary (1913)

Fa \Fa\ (f[aum]), n. [It.] (Mus.)
      (a) A syllable applied to the fourth tone of the diatonic
          scale in solmization.
      (b) The tone F.

Source : WordNet®

fa
     n : the syllable naming the fourth (subdominant) note of the
         diatonic scale in solmization