n.1 a speech sound in one syllable in which the articulation begins as for one vowel and moves as for another (as in coin, loud, and side).
2 a a digraph representing the sound of a diphthong or single vowel (as in feat). b a compound vowel character; a ligature (as {aelig}).
Derivatives:
diphthongal adj.
Etymology: F diphtongue f. LL diphthongus f. Gk diphthoggos (as DI-(1), phthoggos voice)