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Impressions are the number of times an ad was displayed. Impressions refer to the number of times an ad (or group of ads) was displayed on a webpage, in an app, or in any other medium like video.

When a user loads a page and sees 3 ad slots in a website, this means that the user saw 3 impressions. If these ads had multiple ads inside, via a ad network, each ad from each advertiser correspond to one impression for each advertiser.

From The PPC Web: