CPM, or cost per thousand, is a metric used in advertising to measure the cost of reaching one thousand potential customers or viewers. It is calculated by dividing the total cost of an ad campaign by the number of impressions, or views, it receives, and then multiplying that number by one thousand. CPM is often used to compare the cost-effectiveness of different advertising strategies and can help businesses make informed decisions about their marketing budget. Additionally, CPM can also refer to the amount an advertiser pays for one thousand clicks on their ad, making it a valuable tool for online advertising as well. Overall, CPM is an essential factor in determining the success and return on investment of an advertising campaign.