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B-tree (or b-tree)
1. (balanced multiway search tree) of degree n (≥2). A multiway search tree of degree n in which the root node has degree ≥2, every nonterminal node other than the root has degree k, where n/2 ← kn

and every leaf node occurs at the same level. Originally defined by R. Beyer and E. McCreight, the data structure provides an efficient dynamic retrieval device.

An extension to a B-tree is a B+ tree, which is used as a primary index to an indexed file. It comprises two parts: a sequential index containing an entry for every record in the file, and a B-tree acting as a multilevel index to the sequential index entries. B+ trees are used in VSAM.

2. A binary tree with no nodes of degree one.