diphthong

Sound composed of two vowel elements joined to form one sound. English diphthongs include the vowels in ‘nice’, ‘south’, and ‘voice’; also, in many accents, including the standard accent of England, those of ‘face’, ‘nose’, ‘fierce’, ‘scarce’, and ‘tour’.

The term is sometimes also used to refer to sequences of vowel letters, or diagraphs (conventional two-letter sequences with special values), for example, the ea of ‘bread’, pronounced as a single vowel; particularly so when written as a ligature, as æ in ‘Cæsar’.