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.
WordNet (r) 3.0 (2006):
adj 1: of or relating to interstices
The Free On-line Dictionary of Computing (30 December 2018):
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..