What is eCPM?
eCPM (effective cost per mille) is the revenue or cost per thousand impressions, calculated regardless of how the ads were actually priced. Publishers use it to compare earnings from CPM, CPC and CPA deals on the same basis: total earnings divided by impressions, times 1,000. If 20,000 impressions earn $50, the eCPM is $2.50.
eCPM makes different pricing models comparable. An app developer might compare an ad network paying per install with another paying per impression by converting both to eCPM. Ad mediation platforms historically ranked networks by eCPM in the waterfall.
Different platforms sometimes use "RPM" (revenue per mille) for publisher earnings. Whatever the label, check whether invalid impressions and refunds have been removed before comparing.