|Sound change and alternation|
In phonology, hiatus (//; Latin: [hɪˈaːtʊs] "gaping") or diaeresis (// or //, from Ancient Greek διαίρεσις 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 diphthong.
The English words hiatus and diaeresis themselves 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 Zulu, and Polynesian languages such as Hawaiian and Maori. Extreme 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.
Intrusive R 
Some non-rhotic dialects of English insert an /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, French, and English the second of two vowels in hiatus is marked with a diaeresis (or "tréma"). Examples in English include coöperate, daïs and reëlect but 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 name "Zoë".
Other ways 
- abhainn "river" [a.ɪɲ]
- adha "liver" [ɤ.ə]
- ogha "nephew" [o.ə]
- cumha "condition" [kʰũ.ə]
- latha "day" [lˠ̪a.ə]
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.
See also 
- Linking and intrusive R
- Vowel cluster
- Words without consonants – extreme examples of hiatus
- Movable nu
- hiātus. Charlton T. Lewis and Charles Short. A Latin Dictionary on Perseus Project.
- "diaeresis". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005.
- διαίρεσις. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at Perseus Project
- "Voice and Speech in the Theatre"
- diaeresis: December 9, 1998. The Mavens' Word of the Day. Random House.
- Umlauts in English?. General Questions. Straight Dope Message Board.