One of the two general categories used for the classification of speech sounds, the other being consonant. Phonetically, they are sounds articulated without a complete closure in the mouth or a degree of narrowing which would produce audible friction; the air escapes evenly over the centre of the tongue. If air escapes solely through the mouth, the vowels are said to be oral; if some air is simultaneously released through the nose, the vowels are nasal. In addition to this, in a phonetic classification of vowels, reference would generally be made to two variables, the first of which is easily describable, the second much less so: (a) the position of the lips - whether rounded, spread, or neutral; (b) the part of the tongue raised, and the height to which it moves.

Relatively slight movements of the tongue produce quite distinct auditory differences in vowel (or vocalic) quality. Because it is very difficult to see or feel these movements, classification of vowels is usually carried out using acoustic or auditory criteria, supplemented by details of lip position. There are several systems for representing vowel position visually, e.g. in terms of a vowel triangle or a vowel quadrilateral such as the cardinal vowel system.

In establishing the vowel system of a language, several further dimensions of classification may be used. One criterion is in terms of the duration of the vowel (whether relatively ‘long’ or ‘short’ vowels are used). Another is whether, during an articulation, there is any detectable change in quality. If the quality of a vowel stays unchanged, the term pure vowel, or monophthong, is used. If there is an evident change in quality, one talks instead of a gliding vowel. If two auditory elements are involved, the vowel glide is referred to as a diphthong; if three elements, as a triphthong. In the distinctive feature theory of phonology, the term vocalic is used as the main feature in the analysis of vowel sounds.

Yet another way of classifying vowels is in terms of the amount of muscular tension required to produce them: vowels articulated in extreme positions are more ‘tense’ than those articulated nearer the centre of the mouth, which are ‘lax’.