Difference between parameter and argument

Generally, the parameters are what are used inside the function and the arguments are the values passed when the function is called. (Unless you take the opposite view — Wikipedia mentions alternative conventions when discussing parameters and arguments).

double sqrt(double x)
{
    ...
    return x;
}

void other(void)
{
     double two = sqrt(2.0);
}

Under my thesis, x is the parameter to sqrt() and 2.0 is the argument.

The terms are often used at least somewhat interchangeably.

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);