A diphthong is a combination of two adjacent vowel sounds within the same syllable. It is also known as a gliding vowel because the tongue moves during the pronunciation of the vowel. The word "diphthong" originates from Greek, meaning "two voices" or "two sounds". Diphthongs contrast with monophthongs, where the tongue or other speech organs do not move and the syllable contains only a single vowel sound. For example, in English, the word "ah" is spoken as a monophthong (/ɑː/), while the word "ow" is spoken as a diphthong in most varieties (/aʊ/) .

There are eight diphthong sounds in English:

• /ɑι/ (i, igh, y) as in "crime," "light," "byte"
• /eι/ (ea, ey, ay, ai, a) as in "break," "prey," "pray," "braid," "ache"
• /əʊ/ (ow, oa, ough) as in "slow," "moan," "dough"
• /ɑʊ/ (ou, ow) as in "hound," "crown"
• /eə/ (ai, ea) as in "lair," "bear"
• /ιə/ (ee, ie, ea) as in "jeer," "pier," "near"
• /ɔι/ (oy, oi) as in "boy," "oil"
• /ʊə/ (u) as in "pure"

It is important to note that diphthongs are different from disyllabic vowel pairings, which have two distinct syllables.