See also Cost Per Thousand.

Advertisers’ cost per thousand viewers exposed to a commercial. The total cost for one or a series of commercials is divided by the projected audience shown in thousands. If the cost of a commercial is $50,000 and the projected audience is 4,606,000 (4,606,000 divided by 1,000), then the CPM equals $10.86.

Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf