Ad impression

An ad impression is counted each time an ad is loaded to be shown on a screen, whether or not anyone sees it.

Under IAB measurement guidelines an impression is counted when the ad begins to render, not when a person looks at it. An ad loaded at the bottom of a page nobody scrolls still counts.

That gap is why the industry added a second standard, the viewable impression, and why a CPM bought on served impressions costs more per real view than it appears to.