On the 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. Not to be confused with cookie consent pages directives set by the Data Protection Directive from the EU.
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.
In digital marketing, the term "interstitial" is often used in the sense of "interstitial advertising", rather than "interstitial webpage". In some cases, this may lead to confusion because interstitial ads are not always served on interstitial web pages. According to a standard advanced by the IAB, an interstitial (also known as a between-the-page ad) can either be displayed on a separate webpage, or appear briefly as an overlay on the target page. Moreover, mobile advertising guidelines created by the Mobile Marketing Association (MMA) include in-app interstitial ads, that are integrated into applications, rather than web pages.
In January 2017 Google added a search ranking penalty to pages with interstitials. 
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