Diphthong

n. digraph; sound composed of two continuous inseparable elements (Phonetics)

In phonetics, a diphthong (also gliding vowel) (Greek δίφθογγος, "diphthongos", literally "with two sounds," or "with two tones") is a monosyllabic vowel combination involving a quick but smooth movement from one vowel to another, often interpreted by listeners as a single vowel sound or phoneme. While "pure" vowels, or monophthongs, are said to have one target tongue position, diphthongs have two target tongue positions. Pure vowels are represented in the International Phonetic Alphabet by one symbol: English "sum" as , for example. Diphthongs are represented by two symbols, for example English "same" as , where the two vowel symbols are intended to represent approximately the beginning and ending tongue positions.
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Noun
1. a vowel sound that starts near the articulatory position for one vowel and moves toward the position for another
(hypernym) vowel, vowel sound
(derivation) diphthongize, diphthongise

n. diphthong, digraph, sound composed of two continuous inseparable elements (Phonetics)

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

(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.
  

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