| height(left child) - height(right child) | ≤ 1
(If a node does not have a left child, then height(left child) = 0) (If a node does not have a right child, then height(right child) = 0)
Example:
(As we will see, the running time for a findKey(k) operation in a AVL tree is guaranteed to be O(log(n))
(As we will see, the insert and remove operation must re-balance the AVL tree....)