Cost per impression (CPI) and cost per thousand impressions (CPM) are terms used in traditional advertising media selection, as well as online advertising and marketing related to web traffic.
CPI is the cost or expense incurred for each potential customer who views the advertisement(s), while CPM refers to the cost or expense incurred for every thousand potential customers who view the advertisement(s).
In recent years, to address the potential issue of advertisers getting charged for low-value ad impressions, a new metric has become an industry standard: viewable CPM.
[1] Cost per impression is the closest online advertising strategy to those offered in other media such as television, radio or print, which sell advertising based on estimated viewership, listenership, or readership.
When advertising rates are described as CPM or CPI, this is the amount paid for every thousand qualifying impressions served at cost.