An ad that displays in a new browser window behind the current browser window.
The pop-under ad is the sneakier relative of the pop-up ad. While pop-up ads are often shown (and closed) instantly, pop-under ads linger behind the current browser window, appearing only after other windows have been closed.
Pop-under ads are used extensively in advertising on the Web, though advertising is not the only application for pop-under windows.
A pop-under ad is also referred to as a pop-under.