CPM, or cost per thousand impressions, is the marketing world’s metric for judging the merits of different media buys.

Offline, CPM is calculated by taking the total cost of a given ad buy, dividing it by the total estimated viewership of a given advertisement, and multiplying the total by 1000. Here’s an example: You buy a magazine ad for US $5,000. The magazine’s subscriber base is 50,000. Therefore, the CPM will be ($5,000/50,000) x 1,000, or $100.

On the Web, CPM is a little different. Since it’s so difficult to accurately determine the total number of visitors to a website, the CPM is calculated using the number of actual ads served. The distinction is subtle, but critically important: in the offline world, marketers simply guess how many times an ad is seen, whereas on the Web, we know.