NS record
Which nameservers are authoritative for this zone. Delegation lives here.
NS records name the authoritative servers for a zone. The parent zone's NS set, plus glue A/AAAA, is how the rest of the internet finds you. The zone's own NS set should match the parent.
Leftover NS at the registrar after a migration splits the brain: some resolvers hit the old farm, some the new. TTL on NS is often long. Check parent and child, not just one control panel.
See also: SOA record, A record, DNS.