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