Diphthong

Diphthong, two vowel-sounds following one another so closely as to form but one syllable, as in out, where the sound is really composed of a and u. Many double vowels in English are not real diphthongs, there being only one sound heard, whereas some single vowels have a diphthongal sound. The only real English diphthongs are i as in high; i in aye; oi in boil; ow in how; and ew in mew. See SPELLING, LETTERS.