What is Cost Per Mille?

Legal Definition
Cost per mille (CPM), also called cost ‰ and cost per thousand (CPT) (in Latin mille means thousand), is a commonly used measurement in advertising. Radio, television, newspaper, magazine, out-of-home advertising, and online advertising can be purchased on the basis of showing the ad to one thousand viewers. It is used in marketing as a benchmarking metric to calculate the relative cost of an advertising campaign or an ad message in a given medium.

The “cost per thousand advertising impressions” metric (CPM) is calculated by dividing the cost of an advertising placement by the number of impressions (expressed in thousands) that it generates. CPM is useful for comparing the relative efficiency of various advertising opportunities or media and in evaluating the overall costs of advertising campaigns.

For media without countable views, CPM reflects the cost per 1000 estimated views of the ad. This traditional form of measuring advertising cost can also be used in tandem with performance based models such as percentage of sale, or cost per acquisition (CPA).
-- Wikipedia