What is Adsense eCPM?

eCPM stands for "Effective Cost per 1000 impressions"

Impressions are calculated by the amount of times an ad unit is displayed your site.

Do not confuse Ad Impressions with Page Impressions. google offers you the ability to review both.

Page impressions are exactly that, the impression of a page regardless of the number of ad units on that page.

Google's new CPM pricing model for some advertisers has a definite effect on these values and may very well become a key component in the Advertisers filtering capabilities in the near future. Now that Google has CPM pricing the exact value and usage of this data may change but essentially you can use the Google Adsense reporting tools to realize the following:

The basic formula for eCPM is as follows:
Earnings / Impressions = Effective Cost per 1000 impressions

This can be incredibly useful data when looked at either as a whole or more importantly as a custom channel or URL channel report. To further breakdown and maximize the potential for this data as a decision making component in any AdSense campaign I suggest utilizing it in conjunction with the now 200 channels you are allowed with Googles Adsense control panel.

The advantage of knowing the eCPM may not be immediately apparent for a site, it offers a unique insight for a site developing content and traffic over a long period of time. Both Adwords publishers and Adsense advertisers this allows anyone to see the effectiveness of a sites content or ad layout and placement over the duration of a long-term project.

The bottom-line is that the real value of eCPM is best suited for those not looking to get rich quick with adsense, but those more concerned about the long term effects of placement and layout experimentation alongside solid content.