Definition of "diphthong" [diph•thong]

• A complex speech sound or glide that begins with one vowel and gradually changes to another vowel within the same syllable, as (oi) in boil or (ī) in fine. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "A diphthong is the gliding sound of combining vowels, as in the oy in the head-smacking Yiddish oy veh."
• "When one syllable of a word ends with a vowel, and the next syllable begins with the same vowel, the hyphen is placed between the syllables to indicate that the two vowels do not form a diphthong, that is, that they should not be pronounced together."
• "A diphthong is the union of _two_ vowels, pronounced by a single impulse of the voice; as _oi_ in voice, _ou_ in sound."