In web design, a “404 error” or “Not Found” error occurs when a user attempts to access a web page that cannot be found on the server. The server responds with a 404 status code, indicating that the requested page is not available. This error message is displayed to users, informing them that the desired content could not be located. Web designers often customize 404 pages to provide a user-friendly experience, offering navigation options or helpful information to guide users back to the main site or other relevant content.