Syllable
A syllable is a unit of speech that is made up of one or more phones (single sounds or "phonetic segments") and in turn makes up words. It influences the rhythm of a language, its prosody, its poetic meter, its stress patterns, etc.
The general structure of a syllable consists of three parts: the onset, the nucleus, and the coda. The nucleus is usually a vowel or a diphthong; some languages allow consonants like /l/ and /r/ as syllable nuclei. The onset is what comes before the nucleus, and the coda is what comes after it. The nucleus and the coda, together, are sometimes called the rhyme.
Usually only the nucleus is always found. All languages seem to allow syllables with empty codas (i. e. no consonants after the nucleus), and most also allow empty onsets. A syllable of the form CV (consonant + vowel, with an empty coda) is called an open syllable, while a syllable that has a coda (CVC, etc.) is called a closed or checked syllable.
Phonotactic rules determine which sounds are allowed or disallowed in each part of the syllable. English has relatively loose phonotactic restrictions; syllable may begin with up to three consonants (as in string, splash), and end with up to three or four (as in belittled, sixths). On the other hand, other languages are much more restricted; Japanese, for example, only allows /n/ and a generic "lengthening segment" in a coda, and has no consonant clusters at all (the onset is composed of at most one consonant). Hebrew and Arabic forbid empty onsets (the names transliterated as "Israel", "Abraham", "Omar", "Ali" and "Abdullah", among many others, actually begin with semiconsonantic glides or with glottal or pharyngeal consonants).
Syllable structure often interacts with stress. In Latin, for example, stress is regularly determined by the presence or abscence of a coda in the syllable before the last.