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On the World Wide Web, interstitials are web pages displayed before or after an expected content page, often to display advertisements or confirm the user's age (prior to showing age-restricted material). Most interstitial advertisements are delivered by an ad server. Full-screen interstitial ads are referred to as hyperstitials.
Some people take issue with the use of such pages to present online advertising before allowing users to see the content they were trying to access. Less controversial uses of interstitial pages include introducing another page or site before directing the user to proceed; or alerting the user that the next page requires a login, or has some other requirement which the user should know about before proceeding.
Meaning of interstitial 
In this context, interstitial is used in the sense of “in between”. The interstitial web page sits between a referenced page and the page which references it—hence it is in between two pages. This is distinct from a page which simply links directly to another, in that the interstitial page serves only to provide extra information to a user during the act of navigating from one page to the next.
Many interstitial pages are circumvented by NoScript and AdBlock Plus.
See also