An impression is the display of an ad to a user while viewing a web page. A single web page may contain multiple ads. In such cases, a single pageview would result in one impression for each ad displayed. In order to count the impressions served as accurately as possible and prevent fraud, an ad server may exclude certain non-qualifying activities such as page-refreshes or other user actions from counting as impressions. When advertising rates are described as CPM or CPI, this is the amount paid for every thousand qualifying impressions served.
Cost per mille is one of the most common marketing practice used on the internet along with Cost per click (CPC/PPC) and Cost per action (CPA) (including CPL and CPS). CPM advertising is often preferred by publishers because they can be more certain about the revenue they will generate from their website traffic. CPM can be compared with other marketing price strategies by examining their Effective cost per mille (eCPM). eCPM informs the publisher what they would have received if they sold the advertising inventory on a CPM basis by taking into account the Clickthrough rate (CTR) and/or Conversion rate (CVR) of the campaigns.
Cost per mille is the closest online advertising strategy to those offered in other media such as television or print, which sell advertising based on estimated viewership or readership. CPM provides a comparable measure to contrast internet advertising with other media.