What does "diphthong" mean?

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

Additional senses

  1. 2.To form or pronounce as a diphthong; diphthongize. [R.]

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