n.
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.
[Middle English diptonge, from Old French diptongue, from Late Latin diphthongus, from Greek diphthongos : di-, two; see di-1 + phthongos, sound.]
diphthongal diph·thon'gal adj.
The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.
