What's the NXDOMAIN meaning of nslookup?
Solution 1:
As @n8te already commented, it stands for Non-eXistent Domain.
Technically, NXDOMAIN is a valid response to a DNS query, not an error. It just happens to be a negative response: "no, the specific thing you asked for doesn't exist". Nothing ambiguous about it, case closed, end of discussion. It does not mean the resolver library should retry the query with another DNS server.
Such a negative response can even be cached, although the lifetime of negative responses in a DNS cache is usually set to be fairly short.