A directed graph is called an arborescence if, from a given node 
known as the root vertex, there is exactly
one elementary path from 
to every other node 
.
Arborescence
See also
Arboricity, Directed Graph, Root VertexExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Arborescence." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Arborescence.html