HTTP status codes
Also written: status code, status codes, status line, 200, 301, 302, 400, 401, 403, 404, 500, 502, 503, 504, Location header, redirect
The three-digit result in an HTTP response's first line; the first digit says who must act: 2xx success, 3xx look elsewhere, 4xx the client's mistake, 5xx the server's fault. Common ones: 200 OK; 301 moved for good and 302 moved for now (the Location header says where); 400 bad request; 401 log in first; 403 forbidden; 404 not found; 500 the app crashed; 502 Bad Gateway and 503, a server behind a proxy failed; 504 Gateway Time-out, the app behind a proxy is too slow.