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In computer science, an AVL tree is a self-balancing binary search tree, and it was the first such data structure to be invented.

In an AVL tree, the heights of the two child subtrees of any node differ by at most one.

Lookup, insertion, and deletion all take O ( log n ) time in both the average and worst cases, where n is the number of nodes in the tree prior to the operation.

Insertions and deletions may require the tree to be rebalanced by one or more tree rotations.

In other words AVL tree is a binary search tree where the height of the left subtree differs from the height of the right subtree by at most 1 level, if it exceeds 1 level then rebalancing occurs.