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A vowel sound in which your mouth slides from one vowel shape into another inside a single syllable. You do not give the two parts two beats; they are one moving sound. The vowel in main is the usual classroom example: it starts opener and glides toward an “ee” colour. English is full of these — my, go, now, house — though exactly which words have them shifts a little with accent. It is a fact about sound, not about spelling. Two vowel letters written together (ea, oa, ee) may or may not be a diphthong; and a diphthong may be written with one letter. Teachers use the word; everyday speakers just say the word and never name the sound.
雙母音（台灣也說雙元音；大陸較常用雙元音）
組成Ancient Greek δίφθογγος (díphthongos), “two sounds”: δι- / δίς “twice, two” + φθόγγος (phthóngos), “sound.” It came through Late Latin diphthongus and French diphtongue. It pushed out an old English phrase that meant “twofold sound.”
同家族的字
- monophthongsame Greek “sound” piece, but one steady vowel quality instead of a slide
- triphthongsame piece: three vowel qualities sliding in one syllable
原始詞典資料3 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
diphthongnoun/ˈdɪfˌθɒŋ(ɡ)/
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human-scienceslinguisticsphonologyA complex vowel sound that begins with the sound of one vowel and ends with the sound of another vowel, in the same syllable.
- In Lang Belta vowels usually appear preceding or following a consonant (see previous examples), and as of now no occurrence of diphthongs or triphthongs has been attested.
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human-scienceslinguisticsphonologyA diaphoneme realized as a two-target vowel in some but not necessarily all dialects.
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rareA vowel digraph or ligature.
- And he might have written the name, also, with the diphthong æ, as well as the single vowel, in the initial syllable, throughout all the preceding forms.
- An improper diphthong has only one of the vowels sounded; as, ea in heat, oa in coal.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。