In Web advertising, the term impression is sometimes used as a synonym for view , as in ad view . Online publishers offer and their customers buy advertising measured in terms of ad views or impressions. Since a single Web page can contain multiple ads (depending on its design), a site usually registers more ad views per unit of time than Web pages per unit of time. ( Hits is the term for any requested file, including each of a page's images. Although hits are of interest for traffic measurement purposes, they have no significance for advertisers.)
Page or ad impressions are logged in a log that is maintained by the site server. Programs like Web Trends read the log, abstract meaning from it, and generate a report about site usage. Other programs, such as Central Ad, can keep track of all ad impressions that have been sent and how many of these were clicked on by users.