Each of the points that belongs to a graph.
Strictly speaking, a vertex in a graph is always the endpoint of an edge. However, in a broader sense, a vertex of a graph cannot be connected to any of the other vertices.
In this graph, the vertices are the points identified by the letters A, B, C, D, E and F. All vertices are connected to at least one other vertex by an edge:
In this graph, there are two vertices that are not connected to any other vertex: