From Wikipedia:

Cost per mille (CPM), also called cost ‰ and cost per thousand (CPT) (in Latin mille means thousand), is a commonly used measurement in advertising. Radio, television, newspaper, magazine, out-of-home advertising, and online advertising can be purchased on the basis of what it costs to show the ad to one thousand viewers (CPM). It is used in marketing as a benchmark to calculate the relative cost of an advertising campaign or an ad message in a given medium. Rather than an absolute cost, CPM estimates the cost per 1000 views of the ad. This traditional form of measuring advertising success can also be used in tandem with performance based models such as percentage of sale, or cost per acquisition (CPA).