Diphthong

A coalition or union of two vowel sounds pronounced in one syllable; as, ou in out, oi in noise; -- called a proper diphthong.

A vowel digraph; a union of two vowels in the same syllable, only one of them being sounded; as, ai in rain, eo in people; -- called an improper diphthong.

To form or pronounce as a diphthong; diphthongize.

a vowel sound that starts near the articulatory position for one vowel and moves toward the position for another