As explained here:
Domain name servers store information about part of the domain name
space called a zone. The name server is authoritative for a particular
zone. A single name server can be authoritative for many zones.Understanding the difference between a zone and a domain is sometimes
confusing. A zone is simply a portion of a domain. For example, the
Domain Microsoft.com may contain all of the data for Microsoft.com,
Marketing.microsoft.com and Development.microsoft.com. However, the
zone Microsoft.com contains only information for Microsoft.com and
references to the authoritative name servers for the subdomains.The zone Microsoft.com can contain the data for subdomains of
Microsoft.com if they have not been delegated to another server. For
example, Marketing.microsoft.com may manage its own delegated zone.
Development.microsoft.com may be managed by the parent, Microsoft.com.If there are no subdomains, then the zone and domain are essentially
the same. In this case the zone contains all data for the domain.
More details are available here and here.