In phonology, hiatus (/haɪˈeɪtəs/ hy-AY-təs) or diaeresis (/daɪˈɛrəsɪs, -ˈɪər-/ dy-ERR-ə-siss, -EER-;[1] also spelled dieresis or diæresis) describes the occurrence of two separate vowel sounds in adjacent syllables with no intervening consonant.
For example, some non-rhotic dialects of English often insert /r/ to avoid hiatus after non-high word-final or occasionally morpheme-final vowels.
[2] In Greek and Latin poetry, hiatus is generally avoided although it occurs in many authors under certain rules, with varying degrees of poetic licence.
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.
In colloquial speech the examples above drop the second syllable schwa altogether: ziehen [ˈtsiːn], drohen [ˈdʁoːn], sehen [ˈzeːn].