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 variation on the pop-up window is the pop-under advertisement, which opens a new browser window hidden under the active window.
Pop-unders do not interrupt the user immediately and are not seen until
the covering window is closed, making it more difficult to determine
which web site opened them.
