Definition
In computer science, a graph is an abstract data type that is meant to implement the undirected graph and directed graph concepts from mathematics.
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.
A graph data structure may also associate to each edge some edge value, such as a symbolic label or a numeric attribute (cost, capacity, length, etc.).
Source: Wikipedia (license)
External resources
Graph Theory Online Participants