The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

interstitial
    adj 1: of or relating to interstices

The Free On-line Dictionary of Computing (30 December 2018):

interstitial
intermercial
transition ad

    A 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)