A
DNS record type as per
RFC 1035.
NS in this context is short for name server. This basic type of DNS record specifies a name server (or a set of name servers) which will have control over the specified domain.
For example:
lamer.foo.com. IN NS ns1.lamer.foo.com.
lamer.foo.com. IN NS ns2.lamer.foo.com.
Would specify that "ns1.lamer.foo.com" and "ns2.lamer.foo.com" are to be contacted for any DNS queries under the domain lamer.foo.com.