A syllabic consonant or vocalic consonant is a consonant that forms a syllable on its own, like the m, n and l in the English words rhythm, button and bottle, or is the nucleus of a syllable, like the r sound in the American pronunciation of work. To represent it, the understroke diacritic in the International Phonetic Alphabet (IPA) is used, ⟨ ̩ COMBINING VERTICAL LINE BELOW⟩. It may be instead represented by an overstroke, ⟨ ̍ COMBINING VERTICAL LINE ABOVE⟩ if the symbol that it modifies has a descender, such as in .
Syllabic consonants in most languages are sonorants, such as nasals and liquids. Very few have syllabic obstruents, such as stops and fricatives in normal words, but English has syllabic fricatives in paralinguistic words like shh! and zzz.
In many varieties of High and Low German, pronouncing syllabic consonants may be considered a shibboleth. In High German and Tweants (a Low Saxon dialect spoken in the Netherlands), all word-final syllables in infinite verbs and feminine plural nouns spelled -en are pronounced with syllabic consonants. The High German infinitive laufen (to walk) is pronounced ['la?fn?] or (in some accents) even ['la?f] and its Tweants counterpart loopn is pronounced ['l?:?m?]. Tweants scholars even debate whether or not this feature should be incorporated in spelling, resulting in two generally accepted spelling forms (either loopn or lopen).
Many dialects of English may use syllabic consonants in words such as even ['i:vn?], awful ['?:f] and rhythm ['ðm?], which English dictionaries' respelling systems usually treat as realizations of underlying sequences of schwa and a consonant (/'i:v?n/).
In Danish, a syllabic consonant is the standard colloquial realization of combinations of the phoneme schwa /?/ and a sonorant, generally referred to as schwa-assimilation, e.g. katten (the cat) /'kat?n/ = ['k?ad?n?], dame (lady) /'da:m?/ = ['d?æ:m?], cykel (bike) /'syk?l/ = ['syl?], myre (ant) /'my:r?/ = ['my:?], sove (sleep) /'s?:/ = ['s?:?], reje (shrimp) /'ra:j?/ = [':?], huset (the house) /'hu:?s?ð/ = ['hu:?sð].
In all four dialect groups of Norwegian, a syllabic alveolar nasal, /n/, may be heard. It is syllabic when following other alveolar consonants and occurs most often in the definite singular form of masculine nouns (see Norwegian grammar) where the schwa has elided, e.g. bilen (the car) [bi:.ln?], where it was originally [bi:.l?n]. With some speakers, the schwa may be have been reinserted, especially for words already ending in /n/ where the syllabic /n/ may have been entirely elided afterward, e.g. mannen (the man) can either be pronounced like [m?.nn?], [m?n] or [man.n?n]. In addition to this, a syllabic /n/ always occurs in words like vatn (water) [.tn?] and botn (bottom) [b?.tn?]. This syllabification of alveolar nasals also appears in some Swedish dialects[which?]. In all cases where the alveolar sound becomes retroflex, /n/ also becomes retroflex /?/, e.g. barten (the moustache) [b?.] (see Norwegian phonology#Consonants). A contrastively syllabic retroflex /?/ can also be seen in words like baren (the bar) [ba:.] and barn (child) [ba:?]. In some Norwegian dialects, a syllabic alveolar lateral approximant /l/ may be heard in the same circumstances as syllabic /n/, e.g. puddel (poodle) [p?.dl?], though it is not as common as syllabic /n/. A syllabic /l/ may also be heard in Bergen, where a following syllabic /n/ has elided completely, e.g. solen (the sun) [su:.l?]. In dialects that have palatalisation of some alveolar consonants like Northern Norwegian and Trøndersk, the following syllabic /n/ is also palatalised, e.g. ballen (the ball) [b?.].
All of these consonants are sonorants. The only time obstruents are used syllabically in English is in onomatopoeia, such as sh! [ :] (a command to be quiet), sss [s?:] (the hiss of a snake), zzz [z?:] (the sound of a bee buzzing or someone sleeping), and tsk tsk!  (used to express disapproval or pity), though it is not certain how to define what a syllable is in such cases.
Sanskrit ? [r?] and ? [l?] are syllabic consonants, allophones of consonantal r and l. This continues the reconstructed situation of Proto-Indo-European, where both nasals and liquids had syllabic allophones, r?, l?, m?, n?.
Many Slavic languages allow syllabic consonants. Some examples include:
Several Sinitic languages, such as Cantonese and Hokkien, feature both syllabic m ([m?]) and ng () that stand alone as their own words. In Cantonese, the former is most often used in the word meaning 'not' (?, [m]) while the latter can be seen in the word for 'five' (?, ) and the surname Ng (?,  or ?, , depending on the tone), among others.
A number of languages have syllabic fricatives or fricative vowels. In several varieties of Chinese, certain high vowels following fricatives or affricates are pronounced as extensions of those sounds, with voicing added (if not already present) and a vowel pronounced while the tongue and teeth remain in the same position as for the preceding consonant, leading to the turbulence of a fricative carrying over into the vowel. In Mandarin Chinese, this happens for example with s?, sh?, and r?. Traditional grammars describing them as having a "buzzing" sound. A number of modern linguists describe them as true syllabic fricatives, although with weak frication. They are accordingly transcribed ⟨s, ?, and ?⟩ respectively.
However, for many speakers, the friction carries over only into the beginning of the vowel. The tongue and teeth remain where they were, but the tongue contact is lessened a bit to allow for a high approximant vowel with no frication except at the beginning, during the transition. John Wells at University College London uses the detailed transcriptions ⟨sz⟩ for si and ⟨?⟩ for shi (ignoring the tone), with the superscript indicating the "color" of the sound and a lowering diacritic on the z to indicate that the tongue contact is relaxed enough to prevent frication. Another researcher suggests ⟨s⟩ and ⟨⟩ for si and shi, respectively, to indicate that the frication of the consonant may extend onto the vowel. Some speakers have even more lax articulation, opening the teeth and noticeably lowering the tongue, so that s? sh? r? are pronounced [s ], with the same vowel [?] in each case and no r-coloring.
Standard Liangshan Yi has two similar "buzzed" vowels that are described as syllabic fricatives, [, ]. The former may even be trilled .
Sinologists and linguists working in the Chinese analytical tradition frequently use the term apical vowel (? Shéji?n yuán y?n) to describe the sounds above and others like them in various Sino-Tibetan languages. However, this is a misnomer, as the tongue is actually laminal. The nonstandard symbols ⟨? ? ? ?⟩ are commonly used to transcribe these vowels in place of ⟨z? z ⟩ or ⟨C C C?u C⟩, respectively. The term apical vowel should not be taken as synonymous with syllabic fricative, as e.g., the bilabial syllabic fricative  in Liangshan Yi is not pronounced with the tongue.
Berber, Salish, Wakashan and Chemakuan languages have syllabic obstruents in normal vocabulary, such as Nuxálk [p.t.k.ts], [s?.p?s?] "northeast wind", [s?..s?] or [s.s?] "seal blubber", [.q] "wet", [..] "dry", or [nu.jam....] "we () used to (.) sing (nu.jam.)".
In Standard Yoruba, the consonants m and n may be syllabic and carry tone like vowels. However, they can only stand alone as syllables, not being able to stand as syllable nuclei.
In the Baoulé language, the consonant m or n may be syllabic. As a stand-alone word, it means "I" (first person subject pronoun), as in N ti baule [n t bl?] "I speak Baoulé". Its quality varies with the consonant following it, as in M bá aiman [m bá m?n] "I will come tomorrow".
Japanese is frequently described as having a syllabic N, which has its own "syllabic" letter in Japanese kana, but it is actually moraic. The only actual syllabic consonant is a syllabic nasal as an informal variant of un "yeah", similar to syllabic nasals with similar meanings in English.