The price an advertiser pays for 1,000 impressions. Mille means “thousand” in Latin.
Because the actual cost per single impression is very, very small (sometimes as little as $.0015), the rate advertisers pay is calculated in cost per thousand.
CPM = total cost of campaign ÷ total number of impressions x 1,000.
eCPM is used to show what the CPM would have been if the advertiser purchased impressions instead of clicks or conversions.