What is eCPM?

The effective cost per 1000 impressions, also called effective CPM or eCPM, is a commonly used number in advertising programs. It allows you to track your ad performance by comparing the earnings across different pages and sites. Essentially, effective CPM represents your estimated earnings for every 1000 impressions you receive. This can then be compared to other forms of advertising revenue, which may not use such directly comparable figures as cost per click.
Effective CPM doesn't represent your earnings - rather, it's calculated by dividing your earnings by number of page impressions, then multiplying by 1000. For example, if you earned $0.15 from 25 page impressions, then your eCPM would equal ($0.15/25)*1000, or $6.00. If you earned $180 from 45,000 impressions, your effective CPM would equal ($180/45,000)*1000, or $4.00.
This information can be used to compare revenue across channels that may have widely varying traffic - by figuring the earnings per thousand, it's easy to see which channels are performing better. In the examples above, the site or channel which received 25 page impressions could be considered a better performer because it has a higher eCPM.