NS record
Specifies the authoritative nameservers for a domain. Set at the registry level (via your registrar) and at the parent zone — they have to match.
How it works
When a resolver looks up example.com, it asks the .com registry which nameservers are authoritative; the registry returns the NS records the registrar published. The resolver then asks those nameservers for the actual A/MX/TXT records.
Propagation
NS changes can take up to 48 hours to propagate globally because resolvers cache the previous delegation.