Whats is pop-under?

A type of window that appears behind the browser window of a site that a user visited. In contrast to a pop-up ad, which appears above (above) the browser window, a pop-under is less intrusive as it hides behind other windows. Pop-unders are widely used in web advertising, although advertising is not the only application for pop-under windows.
A pop-under is also referred to as a pop-under ad