A consonant, usually a sonorant, acting as the nucleus of a syllable. An example from English: bottles pronounced as ˈbɒtlz rather than ˈbɒtəlz.
The International Phonetic Alphabet provides a diacritic to indicate that a consonant is syllabic. It is a combining vertical line below (U+0329) as in n̩ m̩ l̩ or for symbols with a descender a vertical line above (U+030D) as in ŋ̍ ɲ̍. The diacritic is often omitted if is clear that the consonant must be syllabic.