Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

interstitial
     adj : of or relating to interstices

Source : Free On-Line Dictionary of Computing

interstitial
     
         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)