Diphthong

In phonetics, a diphthong (also gliding vowel) (from Greek δίφθογγος, "diphthongos", literally "with two sounds" or "with two tones") is a contour vowel—that is, a unitary vowel that changes quality during its pronunciation, or "glides", with a smooth movement of the tongue from one articulation to another, as in the English words eye, boy, and cow. This contrasts with "pure" vowels, or monophthongs, where the tongue is held still, as in the English word papa.

Diphthongs often form when separate vowels are run together in rapid speech. However, there are also unitary diphthongs, as in the English examples above, which are heard by listeners as single vowel sounds (phonemes).