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An Active Directory domain is a collection of objects within a Microsoft Active Directory network. An object can be a single user or a group or it can be a hardware component, such as a computer or printer. Each domain holds a database containing object identity information.
Active Directory domains are grouped in a tree structure; a group of Active Directory trees is known as a forest, which is the highest level of organization within Active Directory. Active Directory domains can have multiple child domains, which in turn can have their own child domains. Authentication within Active Directory works through a transitive trust relationship.
Active Directory domains can be identified using a DNS name, which can be the same as an organization's public domain name, a sub-domain or an alternate version (which may end in .local). While Group Policy can be applied to an entire domain, it is typical to apply policies to sub-groups of objects known as organizational units (OUs). All object attributes, such as usernames, must be unique within a single domain and, by extension, an OU.