Ad Viewability is an advertising metric that tracks impressions that can be actually seen by users on websites and apps.
For example, some ads might record impressions even though a user didn’t scroll to that part of a web page or app to see them and because that can be tracked, advertisers believe that they should not pay for non-viewable impressions.
According to the Media Ratings Council (MRC), a viewable impression measurement of ads in mobile web browser environments should follow the existing guidance for viewability in desktop: 50% of pixels in the viewable space of the browser for a minimum of one second (display) or two seconds (video) for a viewable desktop display impression is a minimum of 50% of pixels (of the digital ad) in view (within the browser) for a minimum of one second.