Is "callback" a keyword? Not as a parameter, but as part of the function's body?

callback is not a keyword, and you're on the right track here. In this example, callback is the name of a parameter where a function will be passed in. Then, in the body of the function, we call the function that was passed in to the callback parameter. We can call it whatever we want.

function add(a, b, fancyFunc) {
    fancyFunc(a + b);
}

add(2, 4, function(sum) {
    console.log(sum); // 6
});

I renamed it fancyFunc and it still works. In this case, we're passing an anonymous function as the argument for fancyFunc. But we could also have passed in a function that has a name.

function add(a, b, fancyFunc) {
    fancyFunc(a + b);
}

function myLogger(val) {
    console.log(val);
}

add(2, 4, myLogger);

I'm declaring a function named myLogger. Then, I'm passing that function in as the 3rd argument to the add function. So myLogger is now in the fancyFunc parameter. In the body of add, it's going to call whatever function is passed in. So by calling fancyFunc it ends up calling myLogger.

Let me know if you have more questions.