Foundations
Status Code
Also: HTTP status, 404, 403, 401
The number in an HTTP reply that says whether the request succeeded, moved, was refused, or broke the server.
In more detail
2xx is success, 3xx a redirect, 4xx a problem with the request (401 not identified, 403 not allowed, 404 not found) and 5xx a server fault. Patterns of codes in logs, such as many 404s or 401s, can reveal scanning and password guessing.