In discrete mathematics, and more specifically in graph theory, a vertex
(plural vertices ) or node is the fundamental unit of which graphs are
formed: an undirected graph consists of a set of vertices and a set of edges
(unordered pairs of vertices), while a directed graph consists of a set of
vertices and a set of arcs (ordered pairs of vertices). In a diagram of a
graph, a vertex is usually represented by a circle with a label, and an edge
is represented by a line or arrow extending from one vertex to another.