tree

Definition

A tree is an undirected graph in which every pair of distinct vertices is connected by exactly one path, i.e. a connected acyclic undirected graph.

In other words, a tree is a connected forest.

References

1. https://en.wikipedia.org/wiki/Tree_(graph_theory)
2. R. J. Wilson, Introduction to graph theory, 4. ed., [Nachdr.]. Harlow Munich: Prentice Hall, 2009, pp. 43-44.