Authoritative Name Server
An authoritative name server is a name server that gives original, first-hand, definitive answers (authoritative answers) to DNS queries and not just cached answers that were obtained from another name server. Therefore it only returns answers to queries about domain names that are installed in its configuration system.
An authoritative name server can either be a master server or a slave server. A master server is a server that stores the original (master) copies of all zone records. A slave server uses an automatic updating mechanism of the DNS protocol in communication with its master to maintain an identical copy of the master records.
Every domain name must be assigned a set of authoritative name servers that are installed in NS records in the parent domain.
For domain names that are registered with a domain name registrar to be installed at the domain registry of a top level domain it is customary and usually required to specifically designate a primary name server and at least one secondary name server. This requirement aims to make the domain still functional even if one name server becomes inaccessible or inoperable.[4] The designation of a primary name server is solely determined by the priority given to the domain name registrar. For this purpose generally only the fully qualified domain name of the name server is required, unless the servers are contained in the registered domain, in which case the corresponding IP address is needed as well.
Primary name servers are often master name servers, while secondary name server may be implemented as slave servers.
Authoritative answer
An authoritative server indicates its status of supplying definitive answers, deemed authoritative, by setting a software flag (a protocol structure bit), called the Authoritative Answer (AA) bit in its responses. This flag is usually reproduced prominently in the output of DNS administration query tools (such as dig) to indicate that the responding name server is an authority for the domain name in question