eCPM stands for “effective cost per mille” ("mille" comes from a Latin word that means thousand, so eCPM means “the effective cost per thousand”).

In other words, eCPM is the effective (or earnings) cost per thousand ad impressions.

eCPM is calculated by taking the total earnings divided by the number of impressions, multiplied by 1000.
eCPM = (Estimated earnings / Number of page views) * 1000

It's a performance indicator for your traffic and it can be used to optimize the success of an ad-based monetization strategy.

CTR (Clickthrough rate) is calculated as (Clicks / Impressions) x 100 and it measures the effectiveness of your advertisement.

eCPC (Effective Cost Per Click) is calculated as (Conversions / Clicks) and helps monitor the profitability of your clicks on a CPC model.