In the Domain name system (DNS), a “zone” is a delegated responsibility for the authoritative value of a resource record. The zone corresponds to a domain name suffix. The DNS root server is responsible for the null suffix; its immediate children are the DNS nameservers for the Internet’s Top-level domain (TLD)s.
A zone is created by storing an NS record whose key is the zone’s subdomain, and whose value is the domain name of the nameserver. (Yes, you have to do DNS resolution on DNS nameservers in order to use them!)
When a nameserver delegates authority to another nameserver via an NS record, it implicitly becomes both a DNS nameserver and a DNS resolver.