An HTTP error message returned to the Web browser by the Web server when a requested page is not available or is restricted. The 4xx is a failure prefix and the 04 means not found, but because 404 is the area code for Atlanta, Georgia, people call the 404 the "moved to Atlanta" code (see
HTTP return codes).
However, the site's designer can create a more friendly message, and they often do to keep the user on their site. Many websites display a simple "Not Found" message. See
500 error and
link rot.