In phonetics and phonology, the nucleus (sometimes called peak) is the central part of the syllable, mostly commonly a vowel. In addition to a nucleus, a syllable may begin with an onset and end with a coda, but the only part of a syllable that is mandatory is the nucleus. The nucleus and coda form the rime of the syllable.

Here are some examples of nuclei:

• cat -
• bed - /e/
• tore - /o/

• ode - /o/
• eat - /i/ (the "ea" part)
• tea - /i/ (the "ea" part)

• see - /i/ (the "ee" part)
• oh - /o/

• fiddle - /i/ and /l/
• bitten - /i/ and /n/ (The /n/ is only for speakers who "swallow" the /t/ sound)

Diphthongs and triphthongs can also serve as the nucleus. Syllables with short vowels as nuclei are sometimes referred to as "light syllables" while syllables with long vowels, diphthongs, or triphthongs as nuclei are referred to as "heavy syllables."

Sonorant consonants such as liquids (such as /r/ and /l/) and nasals (such as /m/ and /n/) can serve as the nucleus if there is no vowel. The nuclei of the last syllable of the final two examples above are examples of such sonorant nuclei. Some languages allow other sounds such as stops to become nuclei.