A CPM auction, also known as a cost-per-mille auction, is an online advertising model where advertisers bid for ad space based on the number of impressions, or views, their ad will receive. The term “mille” refers to one thousand impressions, and the winning bidder pays a set price for every thousand impressions their ad receives. This type of auction allows advertisers to reach a large audience while only paying for the actual views their ad receives, making it a cost-effective option for online advertising. CPM auctions are commonly used in display advertising, such as banner ads, and are a popular choice for businesses looking to increase brand awareness and reach a wide audience.