A NameServer that is authoritative for a domain, and holds the actual ZoneFile of that domain that is edited by the SysAdmin.

It is possible to have more than one primary name server for a domain, provided you have some way of ensuring that their copies of the zone files are always in sync. The more usual way to configure multiple authoritative name servers is to have one primary and one or more secondaries, with the latter obtaining their copies of the configuration using ZoneTransfers.