Originally, in ancient accounts of Greek and Latin, of a minimal unit of speech that could be produced on its own and could, on its own, form a syllable: e.g. [iː] in Latin could form the one-syllable word i ‘go!’. Now, more generally or more precisely, of one that is produced with open approximation and that characteristically forms the nucleus (2) of a syllable: e.g. [a] in bat [bat], [iː] in bee [biː], [ɑː] in are [ɑː]. Distinguished as such from syllabic consonants: e.g. [l] as a nucleus in battle [ˈbatl̩]. Also from semivowels, e.g. [w] as the onset of a syllable in we [wiː], or approximants.
See vocoid for a proposed distinction between a unit defined in purely phonetic terms and one defined phonologically. See vocalic (2) for [± vocalic], as a feature varying independently of [± consonantal]. This can also be distinguished, in a similar account, from [± syllabic].