Cost Per Thousand - CPM

Web sites that sell advertising will guarantee an advertiser a certain number of impressions quoted at X dollars per CPM.

Investopedia Says:
For example, a Web site that has a CPM rate of $20 and guarantees advertisers 100,000 impressions, will charge $2,000 ($20 x 100).

Why the M? It's the roman numeral for 1000.

Cost of reaching an audience on a per-thousand basis. media planners use cost per thousand as a basis for comparison of the cost of advertising in various media. However, this comparison is only valid if the various media all reach the same target audience. The cpm is computed by multiplying the advertising cost times 1000 and dividing by the total audience.