digraph

from 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}]

from 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]