Real Dictionary

diphthong
countable noun: A diphthong is a vowel in which the speaker's tongue changes position while it is being pronounced, so that the vowel sounds like a combination of two other vowels. The vowel sound in ' tail' is a diphthong.

diphthong in American English
a complex vowel sound made by gliding continuously from the position for one vowel to that for another within the same syllable, as ( (aʊ) ) in down; ( (aɪ) ) in ride; ( (ɔɪ) ) in boy; ( (oʊ) ) in boat
noun: an unsegmentable, gliding speech sound varying continuously in phonetic quality but held to be a single sound or phoneme and identified by its apparent beginning and ending sound, as the oi- sound of toy or boil

diphthong in British English
noun: a vowel sound, occupying a single syllable, during the articulation of which the tongue moves from one position to another, causing a continual change in vowel quality, as in the pronunciation of a in English late, during which the tongue moves from the p