digraph (n.)

1788, in linguistics, "two letters used to represent one sound," from Greek di- "twice" (from PIE root *dwo- "two") + -graph "something written," from Greek graphe "writing," from graphein "to write, express by written characters," earlier "to draw, represent by lines drawn" (see -graphy). In mathematics (by 1955) it is a contraction of directed graph. Related: Digraphic.