Wiktionary
polytree
n. (context graph theory English) a graph with at most one undirected path between any two vertices. In other words, a directed acyclic graph (DAG) for which there are no undirected cycles either.
Wikipedia
Polytree
In mathematics, and more specifically in graph theory, a polytree (also known as oriented tree or singly connected network) is a directed acyclic graph whose underlying undirected graph is a tree. In other words, if we replace its directed edges with undirected edges, we obtain an undirected graph that is both connected and acyclic.
A polytree is an example of an oriented graph.
The term polytree was coined in 1987 by Rebane and Pearl.