Definition:Reachability Relation/Definition 2

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

Let $\RR$ be the relation on $V$ defined by letting $x \mathrel \RR y$ $y$ is reachable from $x$.

That is, $x \mathrel \RR y$ there exists a directed walk from $x$ to $y$.

Then $\RR$ is the reachability relation of $G$.