The diaeresis ( dy-ERR-ə-sis, -⁠EER-) is a diacritical mark used to indicate the separation of two distinct vowels in adjacent syllables when an instance of diaeresis (or hiatus) occurs, so as to distinguish from a digraph or diphthong.

It consists of two dots ¨ placed over a letter, generally a vowel; when that letter is an ⟨i⟩, the diacritic replaces the tittle: ⟨ï⟩.

The diaeresis diacritic indicates that two adjoining letters that would normally form a digraph and be pronounced as one sound, are instead to be read as separate vowels in two syllables. For example, in the spelling "coöperate", the diaeresis reminds the reader that the word has four syllables co-op-er-ate, not three, *coop-er-ate. In British English this usage has been considered obsolete for many years, and in US English, although it persisted for longer, it is now considered archaic as well. Nevertheless, it is still used by the US magazine The New Yorker. In English language texts it is perhaps most familiar in the loan words naïve, Noël and Chloë, and is also used officially in the name of the island Teän and of Coös County. Languages such as Dutch, Afrikaans, Catalan, French, Galician, and Spanish make regular use of the diaeresis.