Diphthong /(?; 115, 277)/

Diph·thong

Diphthong

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

Diphthong

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