A metric measuring whether an ad had the opportunity to be seen by a user.
Viewability measures whether an ad was actually viewable on screen, not just served. The IAB standard defines a display ad as viewable if 50% of its pixels are in the viewport for at least 1 second. For video, 50% of pixels must be visible for 2 continuous seconds.
Without viewability tracking, advertisers pay for ads that load below the fold or in background tabs — impressions that have zero chance of influencing a user. Viewability ensures you're paying for actual exposure, not just technical delivery.
