Cost per impression (CPI), or "Cost per thousand impressions" (CPM), is a term used in traditional advertising media selection, as well as online advertising and marketing related to web traffic. It refers to the cost of traditional advertising or internet marketing or email advertising campaigns, where advertisers pay each time an ad is displayed. CPI is the cost or expense incurred for each potential customer who views the advertisement(s), while CPM refers to the cost or expense incurred for every thousand potential customers who view the advertisement(s). CPM is an initialism for cost per mille, with mille being Latin for thousand.