Nucleus
Every syllable has a nucleus, which is the most sonorous part of the syllable. In a typical syllable, the nucleus will be a vowel, produced with an unobstructed vocal tract. The segments that come before the nucleus are called the onset, and if there are any segments after the nucleus they’re called the coda. The nucleus and coda together form a unit that we call the rhyme, and linguists use the Greek letter sigma (σ) to label the entire syllable.
Stabile, C. (2018). Essentials of Linguistics. Pressbooks.