An ad impression is equivalent to an ad exposure.
Ad impressions are the number of times an ad is displayed within a webpage consulted by a visitor. Ad impressions are numbered by ad servers.
Ad impressions measure an Internet advertising campaign volume and power.
An ad impression differs from an ad view because an ad can be displayed below the fold and not be really viewed.
On a publisher site, ad impressions and page views are not equal because a page can have multiple ad placements and sometimes ads are reloaded while the page is not.
The number of ad impressions determine the cost of a CPM campaign or more frequently the allowed budget determine the number of ad impressions.
The total number of ad impressions on a publisher website for a given period is called advertising inventory.
IAB has published guidelines relative to ad impression measurement.