In graph theory, a k-ary tree is a rooted tree in which each node has no more than k children. It is also sometimes known as a k-way tree, an N-ary tree, or an M-ary tree. A binary tree is the special case where k=2.
Types of k-ary trees
- A full k-ary tree is a k-ary tree where within each level every node has either 0 or k children.
- A perfect k-ary tree is a full  k-ary tree in which all leaf nodes are at the same depth.
- A complete k-ary tree is a k-ary tree which is maximally space efficient. It must be completely filled on every level (meaning that each node on that level has k children) except for the last level. However, if the last level is not complete, then all nodes of the tree must be "as far left as possible". 
Properties of k-ary trees
- For a k-ary tree with height h, the upper bound for the maximum number of leaves is .
- The height h of a k-ary tree does not include the root node, with a tree containing only a root node having a height of 0.
- The total number of nodes in a perfect k-ary tree is , while the height h is
Note : A Tree containing only one node is taken to be of height 0 for this formula to be applicable.
Note : Formula is not applicable for a 2-ary tree with height 0, as the ceiling operator approximates and simplifies the true formula, which can be described as
Methods for storing k-ary trees
k-ary trees can also be stored in breadth-first order as an implicit data structure in arrays, and if the tree is a complete k-ary tree, this method wastes no space. In this compact arrangement, if a node has an index i, its c-th child is found at index , while its parent (if any) is found at index (assuming the root has index zero). This method benefits from more compact storage and better locality of reference, particularly during a preorder traversal.
- Storer, James A. (2001). An Introduction to Data Structures and Algorithms. Birkhäuser Boston. ISBN 3-7643-4253-6.