Vowels sounds in English.
In phonetics, a vowel is a sound in spoken language, such as English ah! [ɑː] or oh! [oʊ], pronounced with an open vocal tract so that there is no build-up of air pressure at any point above the glottis. Vowel sounds pronounced together in a single syllable form diphthongs.
Their counterpart sounds pronounced with a closed vocal tract are consonants.
In common usage, vowel is sometimes used to refer to the written representation of vowel sounds (namely the letters A, E, I, O, and U, and certain uses of Y and W*), but in fact English has a very large number of vowel phonemes, and considerable variation in their presence and use from dialect to dialect.