A "404 Error Page" is a standard HTTP response code that indicates a web page could not be found on a 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a URL that does not exist, either because the page has been removed, the URL was typed incorrectly, or the link is outdated.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 404 error occurs, the server typically displays a custom page to inform the user. This page may include suggestions for navigating back to the main site or links to popular pages, helping users find the information they need despite the error.