Domain

In Windows NT/2000/XP and Active Directory, a collection of computers defined by the administrator of a Windows NT/2000/XP Server network that share a common directory database. A domain has a unique name and provides access to the centralized user accounts and group accounts maintained by the domain administrator. Each domain has its own security policies and security relationships with other domains and represents a single security boundary of a Windows NT/2000/XP computer network.

Active Directory is made up of one or more domains, each of which can span more than one physical location. For DNS, a domain is any tree or subtree within the DNS namespace. Although the names for DNS domains often correspond to Active Directory domains, DNS domains should not be confused with Windows 2000 and Active Directory networking domain.