A diphthong (/ˈdɪfθɒŋ/ DIF-thong or /ˈdɪpθɒŋ/ DIP-thong;[1] from Greek: δίφθογγος, diphthongos, literally "double sound" or "double tone"; from δίς "twice" and φθόγγος "sound"), also known as a gliding vowel, is a combination of two adjacent vowel sounds within the same syllable.[2] Technically, a diphthong is a vowel with two different targets: that is, the tongue (and/or other parts of the speech apparatus) moves during the pronunciation of the vowel. In most varieties of English, the phrase no highway cowboy /ˌn ˈhw ˈkbɔɪ/ has five distinct diphthongs, one in every syllable.

American English pronunciation of no highway cowboys, showing five diphthongs: / ɔɪ/

Diphthongs contrast with monophthongs, where the tongue or other speech organs do not move and the syllable contains only a single vowel sound. For instance, in English, the word ah is spoken as a monophthong (/ɑː/), while the word ow is spoken as a diphthong in most varieties (//). Where two adjacent vowel sounds occur in different syllables—for example, in the English word re-elect—the result is described as hiatus, not as a diphthong. (The English word hiatus /ˌhˈtəs/ is itself an example of both hiatus and diphthongs.)

Diphthongs often form when separate vowels are run together in rapid speech during a conversation. However, there are also unitary diphthongs, as in the English examples above, which are heard by listeners as single-vowel sounds (phonemes).[3]