Impression (also known as ad view) is counted every time a creative (ad) is displayed to a user. Impressions are considered fundamental indicators of campaign performance.
Impressions are tracked and reported by ad servers using tracking tags (pixels). If a user refreshes the page and sees the same ad again, it counts as a new impression.
In pricing models, 1,000 impressions are referred to as CPM (cost per mille), where mille is Latin for one thousand. CPM is used instead of individual impressions because the cost per impression is very low compared to clicks or conversions (e.g., $0.002).