Variables are called variables because their values can vary between one execution of the code and the next. This is no different for immutable variables. A non-variable, aka a constant, would be something that has the same value in all executions.
Example:
function circumference(radius)
return 2 * PI * radius
It doesn’t have to be a function parameter. If you read external input into a variable, or assign to it the result of calling a non-pure function, or of calling even a pure function but passing non-constant expressions as arguments to it, then the resulting value will in general also vary between executions of that code.
Note how the term “variable” is used for placeholders in mathematical formulas, despite no mutability going on there. Computer science adopted that term from math.