Definition:Reachable/Definition 2

Definition
Let $G = (V, A)$ be a directed graph.

Let $\mathcal R$ be the reachability relation of $G$.

That is, $\mathcal R$ is the transitive closure of $A$.

Let $u, v \in V$.

Then $v$ is reachable from $u$ iff $u \mathrel{\mathcal R} v$.

Also see

 * Equivalence of Definitions of Reachable