What does diphthong mean?

It is a noun that is used in linguistics and means two vowel sounds joined in one syllable to form one speech sound.

How do you pronounce it?

/ˈdɪfθɔŋ/

or

Where does it come from?

Middle English diptonge, from Middle French diptongue, from Late Latindipthongus, from Greek diphthongos, from di- + phthongos voice, sound

How do you use it?

The sounds of “ou” in “out” and of “oy” in “boy” are diphthongs.