Full Idea
F(x) is a 'function', which indicates the unique value which y takes in
Gist of Idea
'F(x)' is the unique value which F assumes for a value of x
Source
Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0)
Book Reference
Enderton,Herbert B.: 'A Mathematical Introduction to Logic' [Academic Press 2001], p.5