HWND

HWND is a unique handle Windows internally uses to identify each window.  The HWND can be retrieved by some commands (<wininfo>, <win_enumerate>) or is provided by some system variables (_vKeybdFocusWindow_HWND, _vActiveWindow_HWND, _vActiveWindowPrev_HWND).  While some other window attributes like window title or window class are not unique (there can be other windows with the same title or class) the HWND is unique.