triph·thong(trĭf′thông′, -thŏng′, trĭp′-)
A compound vowel sound resulting from the succession of three simple ones and functioning as a unit.
triph·thon′gal (-thông′əl, -thŏng′əl) adj.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.