NS record
A DNS record that lists the authoritative nameservers for a zone. Tells the rest of the internet where to ask about records in this zone.
An NS record points at the authoritative nameservers for a domain or subdomain. It says "for records under this name, ask one of these servers."
example.com. NS ns1.example-dns.com.
example.com. NS ns2.example-dns.com.
Where NS records live
Two layers:
At the parent (the registry). The TLD operator (Verisign for .com) has NS records for example.com pointing at your nameservers. Set when you configure nameservers at your registrar.
At your own zone (the apex). Your DNS host serves NS records at the zone apex pointing back at itself. Usually managed for you when you set the zone up.
Both layers should agree, or weird resolution behavior happens.
Delegating a subdomain
You can NS-delegate a subdomain to a different DNS provider:
sub.example.com. NS ns1.other-provider.com.
sub.example.com. NS ns2.other-provider.com.
Now sub.example.com and everything under it is managed by the other provider. Useful when a team owns just one subdomain and you don't want them in your main DNS zone.
How many NS records you need
Most TLDs require 2 or more. Best practice: 2 minimum, 4 for redundancy at scale. Use nameservers on different networks (e.g., 2 from Cloudflare + 2 from Route 53) if you want to survive a single-provider outage. Most SaaS don't bother; the major DNS providers have very high uptime.