—n. 1. a mark (¨) used as a diacritic over a vowel, as ä, ö, ü, to indicate a vowel sound different from that of the letter without the diacritic, esp. as so used in German. Cf. dieresis. 2. (in Germanic languages) assimilation in which a vowel is influenced by a following vowel or semivowel; apophony.
—v.t. 1. to modify by umlaut.
2. to write an umlaut over.