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Here is the list of AdSense related terms that you should know for sure:

Page impression: A page impression is generally the same as a page view but the number of page impressions can sometimes be different than the page views. This happens when the page is viewed by a visitor but the AdSense code is not executed for some reason (which means that the Ads are not displayed to the visitor).

Ad unit impressions: Whenever you get page impressions, you also get Ad unit impressions. However, the ad unit impressions are dependent on the number of ad units you have on your page i.e. number of ad units that get executed when the page loads on user’s browser. Note that the link unit impressions are also counted as ad unit impressions.