Viewability is an online advertising metric that aims to track only impressions that can actually be seen by users. For example, if an ad is loaded at the bottom of a webpage but a user doesn’t scroll down far enough to see it, that impression would not be deemed viewable.
Viewability is a key currency in digital advertising that measures whether or not an ad had a chance to be seen by a user.
The definition of viewability for video advertising: a minimum of 50% of the ad is in view for a minimum of two continuous seconds. For example, if a video ad is playing but the user is browsing on another tab, that impression wouldn’t be considered viewable.
If you want to get technical – here is a great infographic about it: