It is said that you never get a second chance to make a first impression. Well, in the wonderful world of the World Wide Web, publishers have thousands of chances each day. An impression is counted each time a Web page is shown to a visitor. Advertisers measure the reach of their ads by tracking ad impressions, or the number of times their ads are shown. The revenue that publishers generate based on 1,000 impressions is called CPM.
Impressions are counted each time a page is visited by a user, so a single user can rack up numerous impressions for one website. However, publishers and advertisers are also interested in unique impressions, which count only the number of unique visits to a website. For example, if Greg views 3 pages on a website, while Mary views 4 and Kathy views 10, their visits would total 17 impressions, but only 3 unique impressions. Unique impressions are usually counted by sending a cookie to visitors' browsers that expire in 24 hours. This way, if Greg visits the site on Tuesday morning and Wednesday evening, it would could as two unique impressions.