Argument is often used in the sense of actual argument vs. formal parameter.
The formal parameter is what is given in the function declaration/definition/prototype, while the actual argument is what is passed when calling the function — an instance of a formal parameter, if you will.
That being said, they are often used interchangeably, their exact use depending on different programming languages and their communities. For example, I have also heard actual parameter etc.
So here, x and y would be formal parameters:
int foo(int x, int y) {
...
}
Whereas here, in the function call, 5 and z are the actual arguments:
foo(5, z);