The Collaborative International Dictionary of English v.0.48:

Digraph \Di"graph\, n. [Gr. di- = di`s- twice + ? a writing, ?
   to write.]
   Two signs or characters combined to express a single
   articulated sound; as ea in head, or th in bath.
   [1913 Webster]

WordNet (r) 3.0 (2006):

digraph
    n 1: two successive letters (especially two letters used to
         represent a single sound: `sh' in `shoe') [syn: digraph,
         digram]