Effective Cost per Mille (eCPM) is a unit of measurement from the field of online and mobile advertising.
The eCPM is a very useful unit for online and mobile advertisers. It tells them the alternative cost per mille (CPM) if they bought ad impressions instead of clicks, actions etc. Additionally, advertisers can use this information to contrast revenue across different channels by figuring the earnings per thousand impressions.
So basically, the eCPM is a very suitable unit for advertisers to compare the results of different advertising campaigns.
It is calculated by simply taking the total earnings generated by a campaign and dividing them by the number of impressions in thousands.