Definition of "diphthong"
(rare) A vowel digraph or ligature.
Quotations
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