Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

digraph
     n : two successive letters (especially two letters used to
         represent a single sound: `sh' in `shoe')