Diphthong

/ˈdipˌTHäNG,ˈdifˌTHäNG/

noun

a combination of two vowels forming a compound sound
Out of the simple consonants of the alphabet and our eleven vowels and diphthongs all possible syllables of a certain sort were constructed, a vowel sound being placed between two consonants.
– Hermann Ebbinghaus (1850-1909)