From the HTTP error status code 404, meaning Not Found, returned by a web server responding to a request for a non-existent URL. (Wiktionary)
Page Not Found
You've found our 404 page. This is the page we show you when we can't show you the page you wanted, either because it doesn't exist, or because of some other error. Instead, we're showing you the definition and the etymology for the word '404', and some Dürer rhinos (because we like them).