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A multi-way balanced tree.
The "B" in B-tree has never been officially defined. It could stand for "balanced" or "Bayer", after one of the original designers of the algorithms and structure. A B-tree is _not_ (necessarily?) a "binary tree".
A B+-tree (as used by IBM's VSAM) is a B-tree where the leaves are also linked sequentially, thus allowing both fast random access and sequential access to data.
[Knuth's Art of Computer Programming].