A pointer argument to a function that is treated as an output for the function to write too.
Output Parameters, while abundant in C, are discouraged in modern [[C++]], and should be replaced with a normal return unless absolutely necessary.
```cpp
void square(int& result, int x) {
result = x * x;
}
void main() {
int x = 10;
square(x, x);
std::cout << x << std::endl;
return 0;
}
```