CPM stands for Clickthrough Ratio.
The number of people who click through a link or banner compared to the number of people who view it. If 2 site visitors out of 100 click through a banner, you have a CTR of 2/100=.02 or 50:1 (or 2%).
Note that the CTR refers only to people who clicked through an advertisement or banner. It does not reflect the number of people who performed the Most Wanted Response by registering, subscribing, making a purchase, etc. The latter is known as the conversion rate.
Advertisers usually test banners and other ads to see which draw the largest number of clickthroughs. However, a good CTR does not necessarily mean a good conversion rate. Some viewers will click on a banner out of curiousity. Also, if the publisher's traffic is not targeted to the banner's theme, both clickthroughs and conversion rates will be low.
Banner ads have an extremely low CTR. Over-exposure has blinded the public to their messages.
Advertisements placed in ezines or delivered to opt-in email lists have a higher CTR.
Marketers testing the effectiveness of banners and other advertisements typically use ad tracking software.