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

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "diphthong":
   allophone, alveolar, apico-alveolar, apico-dental, articulation,
   aspiration, assimilation, bilabial, cacuminal, cerebral, check,
   consonant, continuant, dental, dissimilation, epenthetic vowel,
   explosive, glide, glottal, glottalization, guttural, labial,
   labialization, labiodental, labiovelar, laryngeal, lateral,
   lingual, liquid, manner of articulation, modification, monophthong,
   morphophoneme, mute, nasal, occlusive, palatal, parasitic vowel,
   peak, pharyngeal, pharyngealization, phone, phoneme, plosive,
   prothetic vowel, retroflex, segmental phoneme, semivowel, sonant,
   sonority, speech sound, stop, surd, syllabic nucleus,
   syllabic peak, syllable, transition sound, triphthong, velar,
   vocable, vocalic, vocoid, voice, voiced sound, voiceless sound,
   voicing, vowel