- (orthography) A diacritic (¨) placed over a vowel letter (especially the second of two consecutive ones) indicating that it is sounded separately, usually forming a distinct syllable, as in the English words naïve, Noël and Brontë, the French haïr and the Dutch ruïne.
- (linguistics, prosody) The separation of a vowel, often a diphthong, into two distinct syllables.
- (prosody) A natural break in rhythm when a word ends at the end of a metrical foot, in a line of verse.
- The umlaut is an often visually identical diacritic which alters the sound of a single vowel (as in German schön). Properly speaking, the terms diaeresis and umlaut are not interchangeable, though speakers frequently use the term umlaut to refer to a diaeresis.
From Ancient Greek διαίρεσις (diairesis, “division, split”), from διά (dia, “apart”) + αἱρέω (aireō, “I take”).