Diphthong
This is the phonetic sound that results when two vowels are next to each other: “ai” and “ea” and “ou,” for example.
It’s pronounced “diff-thong.”
Why I Looked It Up
I saw a graphic about sounds gone from our alphabet: 12 Symbols From Our Alphabet No Longer Used Today
One of them was the diphthong you still see in Scandinavia where an “a” and an “e” are pushed right up against each other.
It stuck me that I didn’t even know what a diphthong was.