A pop-up is a graphical user interface ( GUI ) display area, usually a small window, that suddenly appears ("pops up") in the foreground of the visual interface. Pop-ups can be initiated by a single or double mouse click or rollover (sometimes called a mouseover), and also possibly by voice command or can simply be timed to occur. A pop-up window must be smaller than the background window or interface; otherwise, it's a replacement interface.
A menu or taskbar pulldown can be considered a form of pop-up. So can the little message box you get when you move your mouse over taskbars in many PC applications.