Effective Cost Per Mille or eCPM (as it is often initialized to) is a phrase often used in online advertising and online marketing circles. It means the cost of every 1,000 ad impressions shown.
CPM is considered the optimal form of selling online advertising from the publisher's point of view. A publisher gets paid every time an ad is shown.
eCPM is used to measure the effectiveness of a publisher's inventory being sold (by the publisher) via a CPA, CPC, or CPT basis. In other words, the eCPM tells the publisher what they would have received if they sold the advertising inventory on a CPM basis (instead of a CPA, CPC, or CPT basis). - from Wikipedia