Lets go back to basic of digital advertising. Define CPM and eCPM?

CPM - The price of 1,000 advertisement impressions on one webpage. If a website publisher charges $2 CPM, that means an advertiser must pay $2 for every 1,000 impressions of its ad. The "M" in CPM represents the roman numeral for 1,000. CPM is the most common method for pricing web ads. However, an advertisement's success cannot be measured by CTR alone, because an ad that is viewed but not clicked on may still have an impact.

eCPM - is a bit of industry shorthand that seems here to stay. You've seen it listed in your AdSense reports; it means, "effective cost per thousand impressions." So how iseCPM calculated, you ask? eCPM is calculated by dividing total earnings by total number of impressions in thousands