404 errors

404 errors are messages indicating that a requested webpage or resource cannot be found on a server.

The number comes from HTTP status codes—404 specifically means the server is reachable but the content doesn't exist at that address.

Links to deleted or moved pages produce 404 errors: Half the links in that old article are 404 errors now.

The term has expanded beyond technical contexts to describe anything missing or unavailable: I texted him but he's 404.