A diphthong is a sound made by combining two vowels, specifically when it starts as one vowel sound and goes to another, like the oy sound in oil.
Diphthong comes from the Greek word diphthongos which means "having two sounds." Notice the di- for "double." So diphthongs are double vowel sounds in words like late, ride, or pout. If two vowels in a row are the same, as in boot or beer, then it's not a diphthong. Linguists, scholars who study language, analyze diphthongs, which differ from language to language. Ironically, the word diphthong has no diphthongs.