1. In PHONETICS, a VOWEL that starts with one quality and moves in the direction of another quality, as in toy, which begins with the quality in lawn and moves towards the quality in pin. The combination is often described as a sequence of two vowels. There are several varieties of diphthong: wide and narrow; closing and opening; centring; falling and rising. A wide diphthong has a marked change in quality: in RP, the vowels in high, how, which move from open to close. A narrow diphthong has less movement: in RP, the vowel of day, which moves from half-close to close. The vowels of weave, groove are narrow diphthongs, because they move slightly within the close vowel area, but this movement is usually disregarded and they are treated as monophthongs. A closing diphthong ends closer than it begins, while an opening diphthong ends more open than it begins. The diphthongs of English tend to be of the closing type: in RP, say, sigh, soy, so, sow. A centring diphthong moves towards schwa: in RP, here, there. In rhotic varieties, this schwa is followed by an r-sound, but not in a non-rhotic variety like RP. A falling diphthong is stressed on the first element, and a rising diphthong is stressed on the second. The diphthongs of English tend to be of the falling type, with the exception of the vowel sound in view, which can be interpreted as rising.
2. Two closely associated letters, such as ai and oy, whether or not they represent a diphthong. Technically these are more accurately known as DIGRAPHS. See MONOPHTHONG, SPEECH.
diph·thong / ˈdifˌ[unvoicedth]äng; ˈdip-; -ˌ[unvoicedth]ông/ • n. a sound formed by the combination of two vowels in a single syllable, in which the sound begins as one vowel and moves toward another (as in coin, loud, and side). Often contrasted with monophthong, triphthong. ∎ a digraph representing the sound of a diphthong or single vowel (as in feat). ∎ a compound vowel character; a ligature (such as æ).DERIVATIVES: diph·thon·gal / difˈ[unvoicedth]änggəl; dip-; -ˈ[unvoicedth]ông-/ adj.
Recorded from late Middle English, the word comes via French and late Latin from Greek diphthongos, from di- ‘twice’ + phthongos ‘voice, sound’.