Viewability

Viewability is a measure of whether an actual human had the opportunity to see an ad. Rather than simply counting that an ad was served, viewability assessment checks whether the ad appeared within the visible area of a browser window — the viewport — and met pre-established criteria around two key dimensions: the percentage of ad pixels visible on screen, and the length of time the ad remained in that viewable space.

Distinguishing a genuinely human-viewed impression from one loaded by a bot or rendered in an off-screen tab allows advertisers to make more meaningful judgements about campaign performance. Without viewability data, raw impression counts can be misleading; an ad technically "served" but never in front of a real user adds no value to a campaign.