1. (n.) A union of two vowel sounds pronounced in one syllable; as, ou in out, oi in noise\; -- called a proper diphthong.

2. (n.) 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.

3. (v. t.) To form or pronounce as a diphthong; diphthongize.

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