Definition:Reachable/Definition 2

Definition
Let $G = \struct {V, A}$ be a directed graph.

Let $\RR$ be the reachability relation of $G$.

That is, $\RR$ is the transitive closure of $A$.

Let $u, v \in V$.

Then $v$ is reachable from $u$ $u \mathrel \RR v$.

Also see

 * Equivalence of Definitions of Reachable