(1):
(n.) 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.
(2):
(v. t.) To form or pronounce as a diphthong; diphthongize.
(3):
(n.) A coalition or union of two vowel sounds pronounced in one syllable; as, ou in out, oi in noise; - called a proper diphthong.