1. Phonetics a complex vowel sound made by gliding continuously from the position for one vowel to that for another within the same
syllable, as ( (aʊ) ; ou) ) in down; ( (aɪ) ; ī) ) in ride; ( (ɔɪ) ; oi) ) in boy; ( (oʊ) ; ō) ) in boat
2. Printing either of the ligatures æ or œ, pronounced as diphthongs in classical Latin
Webster’s New World College Dictionary, Fourth Edition, compiled by the
editors of Webster’s New World Dictionaries. Copyright © 2010 by Houghton
Mifflin Harcourt. Published by Houghton Mifflin Harcourt. All rights reserved.
Word origin of 'diphthong'ME diptonge < LL diphthongus < Gr diphthongos < di-, two + phthongos, voice, sound < phthengesthai, to utter