(1) A data structure accessed beginning at the root node. Each node is either a leaf or an internal node. An internal node has one or more child nodes and is called the parent of its child nodes. All children of the same node are siblings. Contrary to a physical tree, the root is usually depicted at the top of the structure, and the leaves are depicted at the bottom. (2) A connected, undirected, acyclic graph. It is rooted and ordered unless otherwise specified. Formal Definition: (1) A tree is either

• empty (no nodes), or
• a root and zero or more subtrees.