Definition:Directed Graph/Arc/Endvertex/Initial Vertex

Definition
Let $D = \left({V, E}\right)$ be a digraph.

Let $e = u v$ be an arc of $D$, that is, $e \in E$.

The initial vertex of $e$ is the endvertex $u$ which $e$ is incident from.

Also see

 * Definition:Final Vertex of Arc