DNS zone

From Freepedia

A zone in the Domain Name System (DNS) is a subdivision of the entire DNS namespace, from a single point, the zone apex, down either to leaf nodes or to the apices of other zones.

Microsoft's Definition

Also called a "zone of authority", a subset of the Domain Name System (DNS) namespace that is managed by a name server. This administrative unit can consist of a single domain, or it can be a domain that is combined with a number of subdomains. The concepts of a zone and a DNS domain are related: each zone is anchored in a specific domain known as the zone's "root domain"