"Error 404" is a standard HTTP response code that indicates a web page could not be found on the server. This error occurs when a user tries to access a URL that does not exist, either because the page has been removed, the URL was typed incorrectly, or the link is outdated. When a user encounters an "Error 404" message, it typically means that the server is functioning correctly, but the specific content requested is unavailable. Many websites provide a custom 404 page to help users navigate back to the main site or suggest alternative content.