Viewability
Viewability measures whether an ad that was served actually had a chance to be seen: whether it appeared on screen for long enough.
The most widely used standard, from the Media Rating Council and the IAB, counts a display ad as viewable when at least half of its pixels are on screen for one continuous second, and a video ad when they are for two continuous seconds. An ad that loads at the bottom of a page nobody scrolls down to counts as an impression, but not as a viewable one.
Many advertisers now buy by viewable impression (vCPM) or require a minimum viewability rate. Formats that fill the screen well (rich media, in-content video) and well-kept publishers tend to score higher. Together with brand safety, it is one of the basic guarantees that the investment actually reaches people.
