In English phonetics, a vowel is an articulated "speech sound produced without occluding, diverting, or obstructing the flow of air from the lungs (opposed to consonant)" (The Random Dictionary of the English Language, 1987). Phonologically, "a vowel is a speech sound characterized by voicing (the vibration of the larynx) and by absence of obstruction or audible friction in the vocal tract, allowing the breath free passage" (McArthur, 1992, p. 1095).
The quality of a vowel is chiefly determined by the position of the tongue, the lips, the lower jaw, and the resulting size and shape of the mouth and pharynx. Vowel quality is the property that makes one vowel sound different from another. For example, the /i:/ in sheep is different from the /I/ in ship.
In British terminology, vowels are classed as open or closed. In American terminology, they are classed as low or high, according to whether the tongue is held close to the roof of the mouth or low in the mouth. They are classed as front or back in both terminologies, according to whether the body of the tongue is pushed forward or pulled backward. They are classed as rounded or spread, according to the shape of the lips. For example, the /i:/ in sheep is a closed front spread vowel, and the /I/ in ship is a semihigh front unrounded vowel.
Vowel sounds divide into monophthongs (single vowel sounds that may be long or short), diphthongs (double vowel sounds forme