Viewable impression is an online advertising indicator or standard created to take into account the ad space viewability issue on webpages.
Ad viewability is a problem because when an ad is served below the fold, it’s counted as an impression at the adserver level, even if the ad is not viewed. On some ad networks viewability can really be a big issue.
According to MRC and IAB, an ad impression is considered viewable when 50% of its pixels are in view on the screen for at least one second. It applies for the standard display ad.
For IAB Rising Stars (large ad formats that measure in excess of 242,500 pixels) only 30% of pixels must be in the view for a minimum of one continuous second to qualify as viewable.
Standards have still to be established for video, mobile and tablet.
Impresson viewability can be measured by a dedicated code inserted within an ad tag or with the creative code.
Some publishers offer system guaranteeing that ads served are positioned so they are viewable by website visitors.
Viewable ad impression illustrated by Adtech :