Interstitial (interstitial ad) is a full screen ad covering the whole screen within a mobile app. Those ads are placed at natural pauses or after level achievements to avoid annoying app users. Interstitial ads can be static banners or dynamic ads allowing interaction like playable ads. 

They are preferred by app developers because they are better than banner ads in terms of their size and the area they have to convince users. Placement, frequency, and the starting points of interstitial ads are important for conversion optimization.