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.
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