In phonology, hiatus, diaeresis (), or dieresis is the separation of the sounds of two consecutive vowels that occur in adjacent syllables. When, instead, the sounds of two adjacent vowels combine into one syllable with no separation, the result is called a synaeresis. In either case, no consonant intervenes between the two vowels. Both diaeresis and synaeresis may occur either within a word or across word boundaries. Both are particular forms of vowel sandhi, which describes how sounds combine.
Some languages do not have diphthongs, except sometimes in rapid speech, or they have a limited number of diphthongs but also numerous vowel sequences that cannot form diphthongs and so appear in hiatus. That is the case of Japanese, Nuosu, Bantu languages like Swahili, Lakota, and Polynesian languages like Hawaiian and Māori. Examples are Japanese aoi (あおい) 'blue/green', Swahili eua 'to purify', and Hawaiian aea 'to rise up', all of which are three syllables.
Many languages disallow or restrict hiatus and avoid it by deleting or assimilating the vowel or by adding an extra consonant.
A consonant may be added between vowels (epenthesis) to prevent hiatus. That is most often a semivowel or a glottal, but all kinds of other consonants can be used as well, depending on the language and the quality of the two adjacent vowels. For example, some non-rhotic dialects of English often insert /r/ to avoid hiatus after non-high word-final or occasionally morpheme-final vowels.
In Greek and Latin poetry, hiatus is generally avoided although it occurs 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 a change in spelling).
In Dutch and French, the second of two vowels in hiatus is marked with a diaeresis (or tréma) if otherwise that combination could be interpreted as a diphthong or as having one of the vowels silent. Examples are the Dutch word poëzie ("poetry") and the French word ambiguë (feminine form of ambigu, "ambiguous"). This usage is occasionally seen in English (such as coöperate, daïs and reëlect) but 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.
In German, hiatus between monophthongs is usually written with an intervening h, as in ziehen [ˈtsiː.ən] "to pull"; drohen [ˈdʁoː.ən] "to threaten". In a few words (such as ziehen), the h represents a consonant that has become silent, but in most cases, it was added later simply to indicate the end of the stem.
Similarly, in Scottish Gaelic, hiatus is written by a number of digraphs: bh, dh, gh, mh, th. Some examples include abhainn [ˈa.ɪɲ] "river"; latha [ˈl̪ˠa.ə] "day"; cumha [ˈkʰũ.ə] "condition". The convention goes back to the Old Irish scribal tradition, but it is more consistently applied in Scottish Gaelic: lathe (> latha). However, hiatus in Old Irish was usually simply implied in certain vowel digraphs óe (> adha), ua (> ogha).
Correption is the shortening of a long vowel before a short vowel in hiatus.