A tree is a diagram that consists of nodes and lines. The lines make up paths that join the nodes of the tree. The paths never loop back on themselves.

There are many different uses for trees in mathematics. The picture on the right shows a factor tree, and the picture below shows a probability tree.

The top node of a tree is called its root. The lines coming out of each node are called branches. If a node doesn't emit any branches, it is called a leaf.

A node that emits edges connecting to other nodes is called the parent of those nodes. The nodes it is connected to are called its children.