The Collaborative International Dictionary of English v.0.48:

Triphthong \Triph"thong\, n. [Pref. tri- + -phthong, as in
   diphthong: cf. F. triphthonque.] (Orthoepy)
   A combination of three vowel sounds in a single syllable,
   forming a simple or compound sound; also, a union of three
   vowel characters, representing together a single sound; a
   trigraph; as, eye, -ieu in adieu, -eau in beau, are examples
   of triphthongs.
   [1913 Webster]