When we speak about eCPM, we are talking about "effective CPM or effective cost per millle". eCPM allows us to compare media pricing methods by converting everything to a common metric. This is helpful when your mix of ad buys consists of different pricing methods, and you're looking for a way to normalize the prices for comparison sake.
On the publisher side eCPM is used to refer to revenue per thousand impressions. If you know the cost (or revenue) data, and total impressions from an ad, you're able to calculate eCPM using the formulas below.
eCPM = (Total cost from an ad / Total impressions) x 1000
eCPM = (CPC * clicks) / (impressions / 1000)
eCPM = (CPL * leads) / (impressions / 1000)