A diphthong occurs when there are two separate vowel sounds within the same syllable. It is also known as a “gliding vowel,” because one sound literally glides into another.

Some of the first diphthongs that are learned (and some of the most common) are the “oi” sound, like in boil, “oy” like in toy, the “ow” sound like in cow, “ou” like in pout, and “aw” like in raw.

More information about diphthongs:

The Difference Between Diphthongs and Digraphs
List of Diphthong Words