An online ad that appears in a pop-up browser window while the primary Web page is still loading. The word "interstitial" literally means "a small space in between things," which is how these ads appear, as mini-browsers, either on top of or behind your active screen. They are likely to contain large graphics, streaming media, or applets, elements that regular ad banners don't usually have, due to size restrictions. Some studies say that more users click on interstitials than on ad banners. However, popular opinion indicates that users do not like interstitials because they slow down access to destination pages, they launch another browser (thereby slowing your computer down), and they are intrusive (most people don't appreciate having a window suddenly appear on their screen without their permission).
NetLingo Classification: Online Marketing