What is the cost per impression?
Cost per impression (CPI) refers to the rate at which an advertiser agreed to pay for every 1,000 views of a particular ad. A website that serves ads based on CPI doesn't need to click the ad. Each display of the ad in front of a user counts as an impression. The advertiser agrees to pay the website a certain price for every 1,000 impressions the ad receives.
The cost per impression is also known as cost-per-1000 or CPM (the letter 'M' is the Roman number for 1,000).
A CPI agreement is more common with large websites that are a branding opportunity for advertisers. CPI follows a pricing model that is closer to the print style of ad sales. Advertisers pay a set price just to run their ads. The website's ad server monitors the number of impressions and usually adjusts the display rate based on the desired spend of a given advertiser on a monthly or quarterly basis.