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