DNS record types
What is a NS record?
NS records name the servers that are authoritative for a zone.
NS records appear twice: at the parent, where they create the delegation, and inside the zone itself. The parent's copy is the one that actually routes queries; the copy inside the zone is what the nameservers themselves report.
When those two sets disagree, resolvers can behave inconsistently — some queries reach the right servers and some do not, which produces the maddening class of fault where a site works for you and not for a colleague.
Example
example.com. 172800 IN NS ns1.example.com. example.com. 172800 IN NS ns2.example.com.
The mistake people make
Publish at least two nameservers, on separate infrastructure. A single nameserver is a single point of failure for everything at once — not just the website, but mail, because senders cannot look up the MX record either.
Check the NS records on a real domain, with everything else that can quietly break.
Check a domain