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.
The pop-under is a type of window that appears behind the browser window of a Web site that a user has visited. In contrast to a pop-up ad, which appears over (on top of) the browser window, a pop-under is less obtrusive as it hides behind other windows. Pop-unders are used extensively in advertising on the Web, though advertising is not the only application
for pop-under windows.
A pop-under is also referred to as a pop-under ad.