1.NounA sound produced by the vocal cords with relatively little restriction of the oral cavity, forming the prominent sound of a syllable.
"Hawaiian has either five or twenty-five vowels, depending on how they are counted."
2.NounA letter or diacritic representing the sound of a vowel; in English, the vowels are a, e, i, o, u, y (sometimes), and w (rarely).
"The word facetiously is spelled with all six vowels in alphabetical order.; The word cwm, borrowed from Welsh, is an example of the letter w serving as a vowel in English."
3.VerbTo add vowel points to a consonantal script (e.g. niqqud in Hebrew or harakat in Arabic).