Tree Terminologies

graph TD A --> B A --> C A --> D B --> E B --> F F --> J F --> K J --> M D --> G D --> H D --> I H --> L L --> N L --> O

Root: A node that has no incoming edge (no parent).
Node $A$ is the root of the above tree.
Parent: A node $m$ is the parent of nodes $n_i$ if there are edges from $m$ to each $n_i$.
Node $D$ is the parent of nodes $\{G, H, I\}$.
Child: A node $n$ is a child of node $m$ if there is an edge from $m$ to $n$.
Siblings: Nodes $\{n_i\}$ are siblings if they share the same parent node $m$.
For example, $\{G, H, I\}$ are siblings.
Descendants: All nodes that can be reached from a given node by following downward edges (recursively).
Nodes $\{J, K, M\}$ are descendants of node $F$.
Ancestors: All nodes on the path from a given node to the root (excluding the node itself).
For example, ancestors of $M$ are $\{J, F, B, A\}$.
Degree of a Node: The number of children of that node (i.e., number of outgoing edges).
Degree of node $D$ is $3$.
Degree of a leaf node is $0$.
Degree of a Tree: The maximum number of children any node can have in a tree.
Binary Tree has degree $2$ (i.e., each node in the tree has degree $\leq 2$).
Internal Node: A node with at least one child (degree $\geq 1$).
Nodes $\{A, B, D, F, H, J, L\}$ are internal nodes.
External Node (Leaf Node): A node with no children (degree $0$).
Nodes $\{C, E, G, I, K, M, N, O\}$ are leaf nodes.
Level: The level of a node is defined as the number of edges from the root to that node.
Root is at level $0$.
Node $F$ is at level $2$.
Node $M$ is at level $4$.
Height of a Node: The number of edges in the longest downward path from that node to a leaf.
Height of node $F$ is $2$ (path: $F \to J \to M$).
Height of a Tree: The height of the root node.
Height of the above tree is $4$.
Forest: A collection of disjoint trees.
If the root $A$ is removed, the remaining structure forms a forest of three trees rooted at ${B, C, D}$.