The Free On-line Dictionary of Computing (30 December 2018):

balanced tree

    An optimisation of a tree which aims to keep
   equal numbers of items on each subtree of each node so as to
   minimise the maximum path from the root to any leaf node.
   As items are inserted and deleted, the tree is restructured to
   keep the nodes balanced and the search paths uniform.  Such an
   algorithm is appropriate where the overheads of the
   reorganisation on update are outweighed by the benefits of
   faster search.

   A B-tree is a kind of balanced tree that can have more
   than two subtrees at each node (i.e. one that is not
   restricted to being a binary tree).

   (2000-01-10)