A domain is a logical partition of an Active Directory forest. If a forest
consists of multiple domains, then the domain in question is a part of that
active directory. Objects created in that partition will only exist and be
replicated within that partition. Additionally, all Active Directory
forests have two additional partitions that are distributed to all domain
controllers in the forest - the Configuration and Schema.
A domain is also a boundary of "security policy" - but not the ultimate
security boundary it was once considered. The ultimate security boundary is
the Active Directory forest itself.