Click Through Rate (CTR) - The number of ad clicks divided by the number of impressions for the page of ads that appears when a link unit is clicked.

Cost Per Click (CPC) - The CPC is the amount an advertiser (not the website publisher) pays Google each time a user clicks on his/her ad. Google AdWords has a CPC pricing system.

Cost Per Action (CPA) - The cost-per-action (CPA) is the amount an advertiser pays when a user completes a certain action. For example, an airline might pay a certain CPA every time a user clicks on their ad and then purchases a plane ticket.

Effective CPM (eCPM)- From a publisher's perspective, the effective cost-per-thousand impressions (eCPM) is a useful way to compare revenue across different channels and advertising programs. It is calculated by dividing total earnings by the number of impressions in thousands. For example, if a publisher earned $180 from 45,000 impressions, the eCPM would equal $180/45, or $4.00. However, please keep in mind that eCPM is a reporting feature that does not represent the actual amount that you are paid.

Page Impressions  - A page impression is generated every time a user views a page displaying Google ads. Google counts one page impression regardless of the number of ads displayed on that page. For example, if you have a page displaying three ad units and it is viewed twice, you will generate two page impressions and six ad unit impressions.

Looking for more definitions for Adsense Terms?  Read the official Adsenseglossary here