Interstitial essentially means “between” and are the ads which appear before or between webpages (when links are clicked, a user is taken to one of these ad pages before the content they clicked to get to).
Interstitial can also be used to mean a floater/overlay. These are ads that appear on top of the page in the same browser window, effectively floating.
They can be interactive and/or affect the sites appearance, or sometimes are just an image or video playing.
Glossary Index