A digraph (or a directed graph) is a graph in which the edges are directed. (Formally: a digraph is a (usually finite) set of vertices V and set of ordered pairs ( a,b) (where a, b are in V) called edges. The vertex is the initial vertex of the edge and the terminal vertex.
a pair of signs or symbols (two graphs), which together represent a single sound or a single linguistic unit. The English writing system employs many digraphs (for example, th, ch, sh, qu, and so on). The same two symbols may not always be interpreted as a digraph (for example, ca th ode versus ca th ouse).