A concept from graph theory concerned with whether there is a path between two vertices in a directed graph. Vertex V is said to be reachable from vertex U provided that there is a path from U to V. There may be several different paths from one vertex to another, the shortest being called a geodesic. The set of points that can be reached from a given vertex V is called the reachable set of V.

A directed graph is unilaterally connected when, for any pair of vertices, at least one vertex is reachable from the other.