A function object, also called a functor, functional, or functionoid, is a computer
programming construct allowing an object to be invoked or called as if it were 
an ordinary function, usually with the same syntax.

/* Callback function */
int compare_function(int A, int B) {
  return A < B;
}
...
/* Declaration of C sorting function */
void sort_ints(int* begin_items, int num_items, int (*cmpfunc)(int, int) );
...
int main(void) {
    int items[] = {4, 3, 1, 2};
    sort_ints(items, sizeof(items)/sizeof(int), compare_function);
    return 0;
}


C++ could use Fuction instead Callback:

// class that will act as a function
class compare_class {
  public:
  bool operator()(int A, int B) {
    return (A < B);
  }
};
...
// Declaration of C++ sorting function.
template <class ComparisonFunctor>

void sort_ints(int* begin_items, int num_items, ComparisonFunctor c);
...
int main() {
    int items[] = {4, 3, 1, 2};
    compare_class functor;
    sort_ints(items, sizeof(items)/sizeof(int), functor);
}


http://en.wikipedia.org/wiki/Function_object
http://en.wikipedia.org/wiki/Callback_(computer_science)