A balanced binary search tree where the height of the two subtrees (children) of a node differs by at most one. Look-up, insertion, and deletion are O(log n) (see BigONotation), where n is the number of nodes in the tree.

This is done by keeping a value in each node of the tree which is equal to it's height. (ie: for leaf nodes it's 0, for nodes above them it's 1, nodes above them 2 and so on).

When you insert a new node, you see if this makes one subtree off an internal node longer than the other and if so do a rotate so that they are equal again.