An impression is an event that occurs whenever a display ad is served to a viewer of a website. It is a common unit of measurement to reference the number of times a specified ad unit will be shown as part of a display ad campaign. When constructing a CPM-based display ad proposal, the number of impressions is used to determine the cost.
Impressions are generally calculated for individual ad units. For example, a site may have delivered 2,500,000 impressions of a 728×90 leaderboard ad unit during a month and 2,800,000 impressions of a 300×250 rectangle ad unit. If an ad unit appears on every page of a site, the number of impressions should approximately equal the number of pageviews received (see the relevant link below if this isn’t the case).
Impressions are also one of the variables in the equations to determine the CTR or RPM.
In recent years, the concept of a “viewable impression” has become more common. Because the entire height of a webpage is not always viewable, it is possible for some ads to be served (and thus record an impression) without ever being seen by a visitor to that site.
For example, the black line below represents the “fold” above which content is immediately viewable upon page load. If a visitor to this page reads only the first paragraph and then closes out, they won’t see the Nissan ad (the 300×250 rectangle) at the bottom. However, an impression was still recorded:
In this example, the 300×250 ad unit would record an impression, but would not record a viewable impression.
This distinction is relevant only for CPM-based display ad campaigns. In a CPC-based campaign, impressions are less relevant because revenue is earned only when visitors click an ad (though publishers may want to know the rate of clicks-per-viewable impression that their ad units are attracting).