Stands for “Cost Per Thousand” (M is the roman numeral for 1,000). This is the amount an advertiser pays for 1,000 impressions of their ad. For example, if a publisher charges $10 CPM, and your ad shows 2000 times, you will pay $20 for the campaign ($10 x 1000 impressions) x 2. Measuring ad success with CPM is most common in awareness campaigns, where impressions are more important than conversions or clicks.
Source: HSMAI CHDM Study Guide
