From The Collaborative International Dictionary of English v.0.48 [gcide]:
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.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
fa
n 1: the syllable naming the fourth (subdominant) note of the
diatonic scale in solmization