Definition of: diphthong

(dif′thông, -thong, dip′-) noun
1. Phonet. A combination of two vowels in one syllable, whether written with two letters, as oi in coil and ou in doubt, or with a single letter, as i in fine and a in bate.
2. In popular usage, either of the ligatures æ, œ, pronounced as a single sound, but which were true diphthongs in classical Latin. See DIGRAPH, TRIGRAPH. [<F diphthongue <LL diphthongus <Gk. diphthongos <di- two + phthongos sound]
—diph·thon′gal adjective
—diph·thong′ic adjective