B-tree - technical definition

(Balanced-tree) A technique for organizing indexes. In order to keep access time to a minimum, it stores the data keys in a balanced hierarchy that continually realigns itself as items are inserted and deleted. Thus, all nodes always have a similar number of keys.

B+tree is a version of B-tree that maintains a hierarchy of indexes while also linking the data sequentially, providing fast direct access and fast sequential access. IBM's VSAM uses this.