diphthong

noun

1. A complex vowel sound that begins with the sound of one vowel and ends with the sound of another vowel, in the same syllable.

   “In Lang Belta vowels usually appear preceding or following a consonant (see previous examples), and as of now no occurrence of diphthongs or triphthongs has been attested.”

2. A diaphoneme realized as a two-target vowel in some but not necessarily all dialects.
3. (rare)A vowel digraph or ligature.

   “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.”

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

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