The Collaborative International Dictionary of English v.0.48:

Diphthong \Diph"thong\ (?; 115, 277), n. [L. diphthongus, Gr. ?;
   di- = di`s- twice + ? voice, sound, fr. ? to utter a sound:
   cf. F. diphthongue.] (Ortho["e]py)
      (a) A coalition or union of two vowel sounds pronounced in
          one syllable; as, ou in out, oi in noise; -- called a
          proper diphthong.
      (b) A vowel digraph; a union of two vowels in the same
          syllable, only one of them being sounded; as, ai in
          rain, eo in people; -- called an improper diphthong.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Diphthong \Diph"thong\, v. t.
   To form or pronounce as a diphthong; diphthongize. [R.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

diphthong
    n 1: a vowel sound that starts near the articulatory position
         for one vowel and moves toward the position for another