Basics · also called ad impression, imp
Impression
An impression is one instance of an ad being served and counted as displayed to a user; it is the basic unit that most online advertising is bought and sold by.
An impression is one showing of one ad. If you load a page with three ads, that is three impressions; if you reload it, that is three more. Because most online ads are priced per thousand impressions (CPM), the impression is the currency of the whole industry.
Precision matters here. Industry guidelines from the IAB and MRC count an impression when the ad is actually delivered and begins to render on the device (a "rendered" or "begin-to-render" count), not merely when the ad server decides to send it. That is different again from a viewable impression, which requires the ad to be on screen for a minimum time, and from a "measured" impression, where a verification tag was able to check it at all.
Impressions are also what fraudsters fake. Bots, hidden ads, ad stacking and pixel stuffing all generate impressions that no human sees. That is why buyers increasingly pay for viewable, human, filtered impressions rather than raw counts.
Think of it like this
An impression is like a leaflet handed to a passer-by: it counts as delivered even if they never read it, which is why advertisers also want to know whether anyone actually looked.
An example
A news site in Germany serves 2,000,000 impressions of a car ad in a day. If 1,300,000 of them meet the viewability standard, the viewable rate is 65%.
Related terms
CPM (cost per mille)
CPM (cost per mille) is the price an advertiser pays for one thousand ad impressions; "mille" is Latin for thousand.
Viewable impression
A viewable impression is an ad impression that meets a viewability standard, by default the MRC's: 50% of the ad's pixels on screen for one continuous second, or two seconds for video.
Ad server
An ad server is the software that stores ads, decides which ad to show in each slot based on rules and priorities, delivers it, and counts impressions and clicks.
Ad inventory
Ad inventory is the total amount of advertising space a publisher has available to sell, usually counted in impressions across its ad slots, pages, apps or screens.
Invalid traffic (IVT)
Invalid traffic (IVT) is any impression, click or visit not coming from a real person with a genuine chance to see the ad, whether caused by bots, fraud or error.
Sources: Media Rating Council