Definition of "diphthong"

And he might have written the name, also, with the diphthong æ, as well as the single vowel, in the initial syllable, throughout all the preceding forms.

1854, Robert Bigsby, Historical and Topographical Description of Repton, in the County of Derby‎, Woodfall and Kinder, page 47

An improper diphthong has only one of the vowels sounded; as, ea in heat, oa in coal.

1860, Joseph E. Worcester, An Elementary Dictionary of the English Language‎, Swan, Brewer, and Tileston, page 12

The diphthong ae is sounded like ē (§7); that is, it has the sound of ey in they.

1874, Theophilus Dwight Hall, A Child’s First Latin Book‎, John Murray, page 3