In phonology, hiatus, diaeresis is the result of two vowel sounds occurring in adjacent syllables, with no intervening consonant. When two adjacent vowel sounds occur in the samesyllable, the result is instead a synaeresis. The Englishwordshiatus and diaeresis themselves each contain a hiatus between the first and second syllables.
Preference
Some languages do not have diphthongs, except sometimes in rapidspeech, 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, Bantu languages like Swahili, Lakota, and Polynesian languages like Hawaiian and Māori. Examples are Japanese aoi 'blue/green', Swahili eua 'to purify', and Hawaiian aea 'to rise up', all of which are three syllables.
Avoidance
Many languages disallow or restrict hiatus and avoid it by deleting or assimilating the vowel or by adding an extra consonant.
Epenthesis
A consonant may be added between vowels 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 to avoid hiatus after non-high word-final or occasionally morpheme-final vowels.
Contraction
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 and synizesis.
Marking
Diaeresis
In Dutch and French, the second of two vowels in hiatus is marked with a diaeresis. That usage is occasionally seen in English 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 namesZoë and Chloë.
Other ways
In German, hiatus between monophthongs is usually written with an intervening h, as in ziehen "to pull"; drohen "to threaten". In a few words, 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 "river"; latha "day"; cumha "condition". The convention goes back to the Old Irish scribal tradition, but it is more consistently applied in Scottish Gaelic: lathe. However, hiatus in Old Irish was usually simply implied in certain vowel digraphs óe, ua.
