A server that contains a master set of data. When there is a change in the data, the authoritative server is updated. Contrast with "non-authoritative server," which is typically a cache server that retains copies of data that were retrieved as a result of normal user queries. Non-authoritative servers may or may not have the latest version of the data.
Authoritative DNS Server
The authoritative server in the DNS system is the one that knows the actual IP address of a website. See DNS