An advertisement's appearance on an accessed Web page. For example, if the page you're on shows three ads, that's three impressions. Advertisers use impressions to measure the number of views their ads receive, and publishers often sell ad space according to impressions. (It can be tough to know, though, whether an impression really means a visitor saw the ad, since they could be browsing without graphics or might not have scrolled down far enough.) Impressions are tracked in a log maintained by a site server and are often sold on a cost per thousand (CPM) basis.
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