Zone

A connected subset of a domain
defining an administrative perimeter
not necessarily equal to the "domain" of the same name
e.g. "jp" zone != "jp" domain
Name servers
Also called authoritative name servers 
DNS servers that manage data for a zone
A single server may manage multiple zones
Typically, there are multiple name servers for a zone
Some name servers may reside in a different zone
Important name servers
Root servers, gTLD servers
Delegation
passing administrative authority from an upper zone to a lower zone (child)