Interstial in German stands for interstitial, which means in between. In terms of banners and large-scale interstitial banners, an interstitial banner is a pop-up banner that sits in between or on top of a web page and covers the entire browser window.
Interstitial banners are triggered by a page change or a click on a button or a hotspot. The interstitial banner is placed on the browser window after opening and covers the browser for a certain period of time. In some versions, it cannot be clicked away, but disappears automatically after the set period of time. The banner itself can be animated or work as Flash.
Interstitial banners and the superstitial banners that work differently are similar to a television advertisement. They have a high attention, however, they are perceived as annoying, because you have no handling against them.