Height of a Tree Definition

The height of a Tree is the height of the root node or the depth of the deepest node.

OR

The height of a tree is the maximum path length from the root to a leaf.

Read - [Height and Depth of a Node in Tree](height-and-depth-of-a-node-in-tree)

For eg. . A single-node binary tree has height 0, and an empty binary tree has height -1. As another example, the following binary tree has height 3, path [7 -> 12 -> 10 -> 11].

                     7
                  /     \
                3         12
              /         /    \
             2        10      20
                     /  \
                    9    11