What is the name of the other vertex of an undirected edge? Let's say I have an undirected graph with an edge $e = \{a,b\}$ connecting vertices $a$ and $b$.

Taking vertex $a$, what is the name of the other vertex? Is it the "co-vertex"? Or simply the "other vertex"? Or the "neighbouring vertex"?
There's no set phrase; any clear description will do. "The other vertex in&nbsp;$e$", "the other endpoint of&nbsp;$e$", "$b$".

I've never heard of "co-vertex" and "neighbouring vertex" is vague, since $a$&nbsp;might have multiple neighbours.