Measured using script embedded within the ad, viewability is an advertising metric that measures whether an ad appeared in the part of the screen that was seen by the user, rather than whether it was just delivered to the page.
The MRC and IAB standards state that a display ad is considered ‘viewable’ when at least 50% of the ad’s pixels are visible to the user for at least one second. For video ads, at least 50% of the video ad’s pixels must be visible, while it is playing, for at least two continuous seconds.
Viewability is expressed as a percentage of impressions, and it can be priced as a goal or a guarantee (i.e. vCPM – viewable cost per mille/thousand).