Fa

Definition: 

The fourth degree of the scale in the solmization system. In French and Italian usage it has become attached, on the fixed-doh principle, to the note F, in whichever scale it occurs

credit: Oxford Companion to Music

Definition: The fourth tone (subdominant) in a diatonic scale. In the "fixed Do" system, Fa is always F 

Source: OnMusic Dictionary

Fourth scale degree