A vowel is a type of sound for which there is no closure of the throat or mouth at any point where vocalization occurs. Vowels can be contrasted with consonants, which are sounds for which there are one or more points where air is stopped. In nearly all languages, words must contain at least one vowel. While a word can be formed without any consonants – such as the English words I or way – no word may consist of only consonants, without a vowel.

Vowels in many languages are not crucial to the general meaning of the word. Rather, a vowel in these languages – of which many are Semitic languages – acts more to give a specific inflection than to differentiate the word from other distinct words. A parallel of this in English can be seen in the example of dive and dove or lay and lie, in which the core word is the same, but the changed vowel denotes tense. Languages that have this type of structure often do not even mark all of their vowels in written text. Both Arabic and Hebrew are good examples of this, where the marking of many vowels is unnecessary in writing.

Since a vowel refers to a specific type of sound, orthographically some letters may represent a consonant in some circumstances, and a vowel in others. In English we can see this with the letters y and w which are most often used to make consonant sounds, but can also be used to represent vowels. In the case of y, for example, we can compare its use in the words yonder and day. In the word yonder, it acts distinctly as a consonant, with the center of the tongue blocking the flow of air on one side by touching the palette of the mouth – as what is called a palatal approximant. In the word day, on the other hand, it is forming a vowel sound akin to if the word were written in English as dei.

In the case of w, we could look at the words woo and how. In the word woo, the letter is acting as a consonant, with the back of the tongue blocking the flow of air on one side by touching the palette of the mouth – what is called a labiovelar approximant. In the word how, it serves as a vowel, which could be represented in English writing as hao.

In English, there are five letters which always represent a vowel when written: a, e, i, o, and u. These five letters represent more than five vowel sounds, however, depending on the word, or if they are combined with other vowels. Compare the letter a in the words hat and hate as one of many examples.