B- Tree

 A B-tree of order m is an m-way true in which 

1)  All leaves are on the similar level

2)  All internal nodes except the root have at most m-1(non-empty) children and at least [m/2] (non empty children)

3)  The number of keys in every internal node is one less than the number of its children and these keys partition the keys in the children in the fashion of a search tree.

4)  The root has at most m children, but may have as few as 2 if it is not a leaf, or none if the tree having of the root alone.