In digital advertising, an impression is counted each time an ad is served to a user’s web-enabled device, whether that’s on a desktop PC, tablet, phone, or smart TV. Ad servers track and count impressions according to parameters set and agreed by the advertiser/publisher, and ad inventory is usually priced and sold by impressions (see CPM). An impression only refers to the number of times an ad is served – the likelihood that the user actually viewed the ad is covered by the viewability metric.