From Wikipedia, The Free Encyclopedia:
"In computer network communications, the HTTP 404, 404 not found, 404, 404 error, page not found, or file not found error message is a hypertext transfer protocol (HTTP) standard response code, to indicate that the browser was able to communicate with a given server, but the server could not find what was requested. The error may also be used when a server does not wish to disclose whether it has the requested information.
"The website hosting server will typically generate a "404 Not Found" web page when a user attempts to follow a broken or dead link; hence the 404 error is one of the most recognizable errors encountered on the World Wide Web."[...]