What is a tree?

The Microsoft Windows 2003 Active Directory glossary defines a tree as “a set of Windows NT domains connected together through transitive, bidirectional trust, sharing a common schema, configuration, and global catalog. The domains must form a contiguous hierarchical namespace such that if a.com is the root of the tree, b.a.com is a child of a.com, c.b.a.com is a child of b.a.com, and so on.”