In linguistics, a diaeresis or (American English only) dieresis (from Greek diairein, "to divide") is the modification of a syllable by distinctly pronouncing one of its vowels. The diacritic mark composed of two small dots placed over a vowel (¨) to indicate this modification is also called a diaeresis.
In French, Greek, and Dutch, and in English borrowings from them, this is often done to indicate that the second of a pair of vowels is to be pronounced as a separate vowel rather than being treated as silent or as part of a diphthong, as in the word naïve or the name Zoë.
In Spanish and Portuguese, it is used over the vowel u to indicate that it is pronounced in places where that vowel would normally be silent. In particular, the u is usually silent in the letter combinations gue and gui, but in word like vergüenza ("shame") or pingüino ("penguin") the u is in fact pronounced forming a diphthong with the following vowel ([we] and [wi] respectivelly).
In Catalan, diaereses serve two different purposes. Similarly to Spanish, they are used in the groups güe, güi, qüe, and qüi to indicate that the u is in fact pronounced forming a diphthong with the following vowel ([we] and [wi] respectivelly). For example, aigües ("waters"), qüestió ("matter"). Also, similarly to French, diaereses are used over i or u to indicate that they do not form a diphthong with a preceding vowel. For example, veïna [b@'in@] ("neighbour", feminine), diürn [di'urn] ("diurnal").
In Finnish, Hungarian and North Germanic languages (i.e., Danish, Icelandic, Norwegian and Swedish) characters looking similar to German umlauts (ü, ä, ö, and å) are in fact considered as letters of their own merits, despite them representing sounds similar to the corresponding sounds in German. As it is not a case of marking grammatical variation, i.e. of tempus or modus, nor of syllable modification, it is in fact neither a case of umlaut nor of diacritical marking. Hence it ought to be improper to call these characters umlauts, however, no better name is known in English.
Using the ISO-8859-1 character encoding, one can type the letters ä, ë, ï, ö, ü, and ÿ. Dozens more letters with the diaeresis are available in Unicode. Unicode also provides the diaeresis as a combining character.