A diphthong ( /ˈdɪfθɒŋ/ or /ˈdɪpθɒŋ/;Greek: δίφθογγος, diphthongos, literally "two sounds" or "two tones"), also known as a gliding vowel, refers to two adjacent vowel sounds occurring within the same syllable. Technically, a diphthong is a vowel with two different targets: That is, the tongue moves during the pronunciation of the vowel. In most dialects of English, the words eye, hay, boy, low, and cow contain diphthongs.

Diphthongs contrast with monophthongs, where the tongue doesn't move and only one vowel sound is heard in a syllable. 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.

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).

In the International Phonetic Alphabet, monophthongs are transcribed with one symbol, as in English sun [sʌn]. Diphthongs are transcribed with two letters, as in English sign [saɪ̯n] or sane [seɪ̯n]. The two vowel symbols are chosen to represent the beginning and ending positions of the tongue, though this can be only approximate.