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 examples include the vowel sounds in boy, owe, eye, safe, how
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)