What is interstitial?
An interstitial is an ad that covers the full screen or most of it, shown at a natural pause such as between game levels or page transitions, and closed by the user. In OpenRTB, Imp.instl = 1 marks any impression as interstitial. For video, Video.plcmt value 3 also describes interstitial placements that play without surrounding video content.
Also called: interstitial ad, full screen ad, instl, imp.instl.
How it works
Imp.instl has existed since OpenRTB 2.0 and defaults to 0. It sits on the impression, so a multi-format interstitial can offer banner and video together under one flag. Banner interstitials usually list full-screen sizes in Banner.format, such as 320x480 and 480x320 on phones, and buyers choose creatives built for that frame.
Video interstitials carry two signals. Imp.instl = 1 says the slot is full screen, and Video.plcmt = 3 classifies the placement under the IAB video guidelines: no video content, the ad is the focus of the screen, and it fills most of the viewport. In the deprecated Video.placement list, value 5 grouped interstitial with slider and floating players, one reason the old values do not map one to one onto plcmt.
Pitfalls: a full-screen ad sent with instl = 0 and a small size, so buyers bid with the wrong creative; instl = 1 offered with only a 300x250 format; and video interstitials labeled plcmt = 1 (instream) even though no content plays. Rewarded placements are often interstitial too, and Imp.rwdd carries that separate fact.
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Related terms
Every term is in the glossary.