Definition: The price of 1,000 advertisement impressions on one webpage. If a website publisher charges $2 CPM, that means an advertiser must pay $2 for every 1,000 impressions of it's ad. The "M" in CPM represents the Roman numeral for 1,000. CPM is the most common method for pricing web ads. 

CPM is a definition for advertisers, for RPM is the one used for creators.