Diphthong

Diphthong (Greek dis, twice; phthoggos, sound), commonly defined as "the union of two vowels in one sound." Strictly speaking, however, both the vowels are pronounced, and an intermediate sound, technically called a glide, develops in the passage from one to the other, there being no definite break between them. Diphthongs, of course, often arise from simple sounds (cf. German mein, Old German min), and also become simple sounds (e.g. the German ci, from a-e). The term, of course, refers to the spoken rather than the written language.