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

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"; see Syllable weight for more discussion.

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 nucleus of the last syllable in the final example at right is an example of a sonorant nucleus.

In a small number of languages, the onset may also be mandatory, so that vowel-initial syllables are not found. There are also a few languages such as Nuxalk and some dialects of Berber, which have some obstruent-only words. It is difficult to divide such words into syllables using conventions from other languages; it may be that the concept of 'syllable' doesn't apply, or that syllabic nuclei are optional in these languages.