Diphthong, noun = a vowel sound, occupying a single syllable, during the articulation of which the tongue moves from one position to another, causing a continual change in vowel quality. For example, the ou in doubt.

Origin: from Latin diphthongus, from Greek δίφθογγος (diphthongos) – with two sounds, which is made up of δίφυες (diphues) – twofold and φθογγος (phthongos) – sound.

Related words
diphthongize, verb = to make (a simple vowel) into a diphthong

monophthong, noun = a simple or pure vowel

triphthong, noun = a composite vowel sound during the articulation of which the vocal organs move from one position, through another and ending in a third

The Chinese word for diphthong is 二重元音 (èrzhòngyuányin) or 雙元音 (shuangyuányin), which literally mean “two weight vowel” and “twin vowel”. 元音 (vowel) means literally “primary/fundamental/basic sound”. This demonstrates a fundamental difference between English and Chinese: many words from other languages are used in English, and technical, scientific and medical terms are often cobbled together from Greek and/or Latin roots. However in Chinese, there are very few foreign loanwords and most words are made up of native roots. If you didn’t know the meaning of diphthong you could only guess it if you knew Greek, whereas you could probably work out the meaning of 二重元音 even if you had never seen it before.