The HTTP error message returned to the browser by the Web server when a requested Web page is not available. The error occurs because the page no longer exists or is restricted. The 404 error can be modified by the site's Webmaster to a more user-friendly message. Like a telephone book, a link to a Web page (URL) is static. Although software can be used to search each link on a site to ensure the target pages still exist, it cannot automatically find new addresses if the pages are no longer present. See 404, HTTP return codes, 500 error, link rot and moved to Atlanta.