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

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].

1
2
3
4
5
6
7

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