Cost-Per-Impressions (CPI) refers to the rate that an advertiser has agreed to pay per 1,000 views of a particular advertisement. A website that serves ads based on CPI doesn’t need the user to click on the ad – each appearance of the ad in front of a user counts as one impression. The advertiser agrees to pay the website a certain price for every 1,000 impressions the ad receives. Cost per impression is also known as Cost-Per-Thousand (CPM). The letter "M" is the Roman numeral for 1,000.