First of all, a balanced tree need NOT necessarily be a binary tree. For example, a 2-3 tree is one of the most balanced forms of tree structures. However, mostly the balanced trees are implemented as binary trees.

Secondly, a balanced tree is a tree, in which the height of each subtree is dependent on the rest of the subtrees (NOT necessarily the same or differing just by one). For example in a Red-black tree, the height of one subtree can at the most be double that of the other.