A viewing area that is larger than the physical borders of the screen. It allows the user to scroll very large documents or multiple documents side-by-side by moving the mouse pointer (cursor) beyond the edge of the screen. For example, you might look through an 800x600 screen resolution into a 1,600x1,200 virtual screen. See virtual desktop
In this example, the user can see 800x600 pixels at a time and can pan the entire 1600x1200 pixels by moving the mouse beyond the edge of the viewable screen.