An ELT Glossary : Syllable
A word will consist of one or more syllables. Each syllable will be made up of a vowel sound which may or may not have surrounding consonants. For example :Oh! /əʊ/, or /ɔː/, and I /aɪ/ are all syllables which consist of a single vowel sound only. (Whether it is a monopthong, as in or or a diphthong as in the other two examples, makes no difference - it is still considered a single vowel sound).
Alternatively, the vowel may be preceded, followed or both prededed and followed by one or more consonants - eg: go /gəʊ/ CV, art /ɑːt/ VC, peat /piːt/ CVC,
through /θruː/ CCV, slice /slaɪs/ CCVC , trust /trʌst/ CCVCC, depths /depθs/ CVCCC
Notice that it is the phonological realisation which is important in defining the syllable - not the written form. Slice for instance contains two letters which are considered vowels in the written form, but is pronounced as a single syllable - CCVC.
All of the words considered so far are monosyllabic - ie they consist of one syllable only. But obviously, words may consist of more than one syllable, for example: carpet /kɑː•pɪt/ has two (i.e it's bisyllabic); however /haʊ•ev•ə/ has three (i.e it's trisyllabic), and electoral /e•lek•tə•rəl/ has four. Words with more than three syllables are called polysyllabic words.