\begin{definition}[Definition:Reachability Relation/Definition 2]
Let $G = \struct {V, A}$ be a directed graph.
Let $\RR$ be the relation on $V$ defined by letting $x \mathrel \RR y$ {{iff}} $y$ is reachable from $x$.
That is, $x \mathrel \RR y$ {{iff}} there exists a directed walk from $x$ to $y$.
Then $\RR$ is the '''reachability relation''' of $G$.
\end{definition}