In phonetics, a vowel is a sound in spoken language, with two competing definitions. In the more common phonetic definition, a vowel is a sound pronounced with an open vocal tract, so that the tongue does not touch the lips, teeth, or roof of the mouth, such as the English "ah" /ɑː/ or "oh" /oʊ/. There is no build-up of air pressure at any point above the glottis. This contrasts with consonants, such as the English "sh" [ʃː], which have a constriction or closure at some point along the vocal tract. In the other, phonological definition, a vowel is defined as syllabic, the sound that forms the peak of a syllable. A phonetically equivalent but non-syllabic sound is a semivowel. In oral languages, phonetic vowels normally form the peak (nucleus) of many to all syllables, whereas consonants form the onset and (in languages that have them) coda. Some languages allow other sounds to form the nucleus of a syllable, such as the syllabic (i.e., vocalic) l in the English word table [ˈtʰeɪb.l̩] (when not considered to have a weak vowel sound: [ˈtʰeɪb.əl]) or the syllabic r in Serbo-Croatian word vrt [vr̩t] "garden". The word vowel comes from the Latin word vocalis, meaning "vocal" ("relating to voice"). In English, the word vowel is commonly used to mean both vowel sounds and the written symbols that represent them.

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