Definitions for "umlaut"
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.