Source : Webster's Revised Unabridged Dictionary (1913)

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}.

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

Source : WordNet®

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