A diphthong is a sound made by combining two vowels in the same syllable, specifically when it starts as one vowel sound and goes to another.
English has examples such as in joy/join, cow/count, my/mind, say/safe.
Diphthongs refer to vowel sounds, not letters (combinations of two letters are called digraphs). Don’t be confused by spelling – some English words have two vowel letters which don’t represent a diphthong (eg head, four), while others have a single vowel representing a diphthong.
Many Aboriginal languages have diphthongs, which may be represented in spelling by a vowel with either y or w.
e.g., Kunwinjku: ruy (cooked), rowk (all)
Yolngu matha: yow (yes), bäy’ (leave behind)