Cost Per Mille (CPM) is a common metric in the world of digital marketing that is used to calculate the cost of 1,000 ad impressions. The term "mille" comes from Latin and stands for a thousand. CPM is often used to evaluate the profitability and efficiency of an online advertising campaign. It provides advertisers with a standardised benchmark to evaluate different advertising options and platforms.

In the CPM model, the advertiser pays a fixed amount for every 1,000 impressions, regardless of whether the user interacts with the advert or not. This distinguishes CPM from other models such as Cost Per Click (CPC), where payment is only made for actual clicks on the advert, or Cost Per Action (CPA), where the costs are linked to a specific action by the user, such as a purchase.

CPM is often used in display advertising and programmatic buying of ad placements. It is particularly useful for campaigns targeting brand awareness as it provides a broad overview of the visibility of the advert. While CPM provides an easy way to evaluate the cost of an advert's reach, it is important to note that a high number of impressions does not necessarily lead to a higher engagement rate or conversion. Therefore, advertisers often use additional metrics such as click-through rate (CTR) and conversion rate to evaluate the overall success of a campaign.