A graph data structure consists of a finite (and possibly

mutable) set of vertices or nodes or points, together

with a set of unordered pairs of these vertices for an

undirected graph or a set of ordered pairs for a

directed graph. These pairs are known as edges, arcs,

or lines for an undirected graph and as arrows,

directed edges, directed arcs, or directed lines

for a directed graph. The vertices may be part of

the graph structure, or may be external entities

represented by integer indices or references.


[Wikipedia](https://en.wikipedia.org/wiki/Graph_(abstract_data_type))