f(x) + g(x), where they both are non-lineariteis. Gives a linear function. What is g(x) named?

Say we have f(x) such that,

f(x) = x^2 (Just an example maybe any other function too e.g: x^6, x^(-14) etc.)

and we have, g(x) such that,

g(x) = -x^2 (Just an example maybe any other function too e.g: -x^6, (-x^-14), etc.)

Now,

f(x) + g(x) = Linear number line.

My question is, what is the function g(x) know with respect to f(x) know as?
Inverse of f(x)?, Negative of f(x)?. What is the nameing convention?