Also known as: ad view
An ad view is counted every time a creative (ad) is served to a user. The impression is considered one of the basic campaign performance indicators.
Impressions are counted and reported by ad servers which use tracking tags (pixels) for measurement purposes. If a user refreshes the page and sees the exact same ad, then another new impression would be counted.
In pricing models, 1000 impressions are marked as CPM, cost per mille; the word mille is Latin for one thousand. The reason for using CPM instead of individual impressions is that the price per impression is very small compared to clicks or conversions, e.g. $0.002.