Pop-under is a term used to describe a window that opens behind the active program window on a computer screen. For example, a user may be viewing a web page using an Internet browser and a pop-under window can open up behind the window. The pop-under window would not be visible until after the Internet browser window is minimized or closed.
The most common use of pop-under windows is to display advertisements to computer users. They are less intrusive, as they do not show up over top of the active program window. However, users could still click one accidentally when minimizing or closing the active program window, without realizing it.
A pop-under window is similar to a pop-up window, which opens and is displayed over top of the active program window.