CPM, or Cost Per Mille, is a pricing model used in online advertising that refers to the cost an advertiser is willing to pay for 1,000 impressions of their ad. Impressions refer to the number of times an ad is served to users, regardless of whether the user actually sees or engages with the ad. CPM is typically calculated on a per-thousand basis and is used to determine the cost of an ad campaign.

Effective CPM, or eCPM, is a variant of CPM that takes into account the actual revenue generated by an ad campaign, rather than just the cost. eCPM is calculated by dividing the total revenue generated by the number of impressions and multiplying by 1,000. This metric is used to compare the relative effectiveness of different ad campaigns and determine which are the most profitable.