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interstitial in·ter·sti·tial (ĭn'tər-stĭsh'əl)
Relating to or situated in the small, narrow spaces between tissues or parts of an organ.
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..