\begin{definition}[Definition:Adjacent (Graph Theory)/Vertices/Digraph]
Let $G = \struct {V, E}$ be a digraph.
Two vertices $u, v \in V$ of $G$ are '''adjacent''' if there exists an arc $e = \left({u, v}\right) \in E$ of $G$ to which they are both incident.
Otherwise they are '''non-adjacent'''.
\end{definition}