A window that can be displayed on the screen at any time on top of anything that is already on the screen. When the window is removed, the original screen display is restored. These are most often used to display warning messages or to confirm a choice. For example, if you try to save a file with a name that is the same as an existing file, you'll see a pop-up window that asks if you want to overwrite the existing file or change the name. These pop-up windows are often called dialogue boxes.
