What is the Domain in Weblogic server?

An Oracle WebLogic Server administration,
Domain is a logically related group of Oracle WebLogic Server resources. 
Domains include a special Oracle WebLogic Server instance called the Administration Server, which is the central point from which you configure and manage all resources in the domain.
Usually,you configure a domain to include additional Oracle WebLogic Server instances called managed Servers.

Note:
Domain is a logically related group of Oracle WebLogic Server resources that are managed as a single unit.