A multigraph is a graph in which we allow more than one edge to join a pair of vertices. Two or more edges that join a pair of vertices are called parallel edges. Every graph, then, is a multigraph, but not all multigraphs are graphs. Some authors define the concept of a graph by excluding graphs with multiple edges or loops. Then if they want to consider more general graphs the term multigraph is introduced. Usually, such graphs have no loops. Formally, a multigraph $G=(V, E)$ is a pair, where $E=(V^{(2)}, f)$ is a multiset for which $f(x,x) = 0$ and $V^{(2)}$ is the set of unordered pairs of $V$

A multigraph can be used to represent a matrix whose entries are nonnegative integers. To do this, suppose that $A=(a_{ij})$ is an $m\times n$ matrix of nonnegative integers. Let $V=S \cup T$ where $S=\{1, \ldots , m\}$ and $T =\{1', \ldots , n'\}$ and connect vertex $i\in S$ to vertex $j'\in T$ with $a_{ij}$ edges.