In phonology, hiatus (; from Latin hiatus, meaning 'gaping') or diaeresis ( or ; from Ancient Greek [diaíresis] "division") refers to two vowel sounds occurring in adjacent syllables, with no intervening consonant. When two adjacent vowel sounds occur in the same syllable, the result is instead described as a synaeresis.
The English words hiatus and diaeresis themselves each contain a hiatus between the first and second syllables.
Some languages do not have diphthongs, except optionally in rapid speech, or have a limited number of diphthongs but also numerous vowel sequences which cannot form diphthongs and thus appear in hiatus. This is the case of Japanese, Bantu languages such as Swahili, and Polynesian languages such as Hawaiian and M?ori. Examples are Japanese aoi () 'blue/green', Swahili eua 'to purify', and Hawaiian aea 'to rise up', all of which are three syllables.
Many languages disallow or restrict hiatus, avoiding it either by deleting or assimilating the vowel, or by adding an extra consonant.
Some non-rhotic dialects of English insert /r/ to avoid hiatus after non-high word-final (or occasionally morpheme-final) vowels, although prescriptive guides for Received Pronunciation discourage this.
In Greek and Latin poetry, hiatus is generally avoided, though it does occur in many authors under certain rules with varying degrees of poetic licence. Hiatus may be avoided by elision of a final vowel, occasionally prodelision (elision of initial vowel) and synizesis (pronunciation of two vowels as one without change in writing).
In Dutch and French, the second of two vowels in hiatus is marked with a diaeresis (or tréma). This usage is occasionally seen in English (examples include coöperate, daïs and reëlect), but it has never been common and over the last century its use in such words has been dropped or replaced by the use of a hyphen except in a very few publications, notably The New Yorker. It is, however, still common in loanwords such as naïve and Noël and in the proper names Zoë and Chloë.
This convention goes back to the Old Irish scribal tradition (though it is more consistently applied in Scottish Gaelic), e.g. lathe (> latha). However, hiatus in Old Irish was usually simply implied in certain vowel digraphs, e.g. óe (> adha), ua (> ogha).
Correption is the shortening of a long vowel before a short vowel in hiatus.