NXDOMAIN

NXDOMAIN ("non-existent domain") is the DNS response code that says: the name you queried does not exist at all. It's RCODE 3 in the DNS protocol.

When you'll see NXDOMAIN

The domain isn't registered, or has expired
The subdomain you queried doesn't exist (even if the parent domain does)
A typo: exmaple.com vs example.com
Recently-deleted records that haven't fully propagated to the resolver you're using

NXDOMAIN vs NODATA vs SERVFAIL

These three look similar but mean very different things:

NXDOMAIN — the name doesn't exist at all.
NODATA — the name exists, but not the record type you asked for. Looking up an AAAA on an IPv4-only host returns NODATA, not NXDOMAIN.
SERVFAIL — the resolver couldn't get an authoritative answer. Often a DNSSEC validation failure, a broken delegation, or unreachable nameservers.

Diagnosing

If WhereIsDNS shows NXDOMAIN across most resolvers but a few say "ok," the record was probably just created — wait for cached negative responses to expire (governed by the SOA's minimum TTL, typically 1 hour). If most say "ok" but a few say NXDOMAIN, those resolvers may have cached a stale negative response from before the record existed.

Related: TTL · SOA records · resolver.