Term for original function used to derive anti-derivative and derivative

Question

In analyzing a function, $f(x)$, it is often useful or necessary to analyze its antiderivate $F(x)$ and/or its derivative $f'(x)$. Does $f(x)$ have a special name that I can use to refer back to it in the same way that I can refer to $f'(x)$ as 'the derivative' and $F(x)$ as 'the antiderivative'? Just calling $f(x)$ 'the function' is bound to cause confusion. I thought about using 'the output function' or 'the original function' but these are non technical terms.