Interstitial means, generally, "of the the space in between things." In website parlance, an interstitial action is typically what happens to a visitor of a website when that visitor clicks between web pages. A visitor can be shown a popup type of form or advertisement or offer that, when dismissed (or filled in and submitted) then lets visitors continue to the page that they they were trying to get to when they clicked the link or button. An interstitial form is interruptive of the user experience of the site and is therefore not best practice.
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