- Degree of a node: the number of branches associated with the node.
- In-degree branch: directed branch toward the node.
- Out-degree branch: directed branch away from the node.
- The first node is called the root.
- in-degree of the root = 0
- Except the root, the in-degree of a node = 1
- out-degree of a node = 0 or 1 or more.