Cost per impression, often abbreviated to CPI or CPM (Cost per mille) are phrases used in online advertising and marketing related to web traffic. They refer to internet marketing campaigns where advertisers pay for every time that their advert is displayed to a user usually in the form of a banner ad on a website, but can also refer to advertisements in Email advertising.
An impression describes the instance when an advert is downloaded by a user while viewing a web page. A single web page may contain multiple adverts and in such cases a single pageview would result in one impression for each advert displayed to that user. 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, but CPM can be compared with different marketing 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 the more traditional media such as on television or in print which sell advertising based on estimated viewership or readership, and as such is popular with advertisers as it is simple to measure campaigns and can be compared and contrasted with their advertising in other media.