If f is a function defined by the equation y=?f(x), then x is called the what?

Question

 If f is a function defined by the equation
y=?f(x),
then x is called the

Answer 1

The input of f(x).

Answer 2

x is the argument of the function f(x).

Answer 3

X may be called the independent variable, and y may be called the dependent variable.