Interstitial \In`ter*sti"tial\, a. Of or pertaining to interstices; intermediate; within the tissues; as, interstitial cavities or spaces in the tissues of animals or plants. [1913 Webster]
interstitial intermercial transition ad <World-Wide Web> A {World-Wide Web} page that appears before the expected content page. Interstitials can be used for advertising (intermercial, transition ad) or to confirm that the user is old enough to view the requested page, etc.. (2003-07-11)