The euphonic modification of a root vowel sound by the influence of a, u, or especially i, in the syllable which formerly followed.

The term umlaut is used for two closely related notions: 1) a special kind of vowel modification, and a 2) particular diacritical mark - composed of two small horizontal dots placed over a vowel, as in German Öfen. In the Unicode character encoding umlaut is not distinguished from the diacresis.

the orthographic mark (¬) placed directly above a vowel in some languages to indicate a change in vowel pronunciation.

a diacritical mark (two dots) placed over a vowel in German to indicate a change in sound

A type of assimilation in which a back vowel is fronted or a low vowel raised because of the influence of a front vowel in the following syllable.

A diacritic, such as two dots, used over a vowel to indicate a change in vowel sound. (e.g., ä, ë, ï, ö, ü). See also ACCENT CHARACTER DIACRITICAL MARK PUNCTUATION

The umlaut mark (or simply umlaut) and the trema or diaeresis mark (or simply diaeresis) are two diacritics consisting of a pair of dots placed over a letter. When the vowel is an i, the diacritic replaces the tittle. The two diacritics are very similar in appearance, and the distinction between them is not always made.