A standard HTTP response code indicating that the requested page does not exist on the server. For SEO purposes, the server should be configured with a custom 404 page with a brief message explaining the error, branding and a search form or alternate links.
Repairing or redirecting 404 errors on your site is a worthwhile activity for many reasons, especially if the 404 errors are the result of other sites linking to an old or incorrect URL on your site.