A monophthong is a vowel sound that is produced with a single, unchanging position of the tongue and mouth. It's a single, uninterrupted sound.
Here are some examples of monophthongs in English:
* a as in "cat"
* e as in "bed"
* i as in "bit"
* o as in "cot"
* u as in "put"
In contrast, a diphthong is a vowel sound that involves a glide between two vowel positions. It's a single syllable but includes two sounds that blend together. Examples include:
* oi as in "boy"
* ou as in "house"
So, while a syllable is a unit of pronunciation, a monophthong is a type of vowel sound that makes up that syllable.