Definition of: umlaut

(m′lout) noun
1. Ling. a The change in quality of a vowel sound caused by its partial assimilation to a vowel or semivowel (often later lost) in the following syllable; vowel mutation: primarily a phenomenon of the Germanic languages. English plurals showing internal vowel modification, such as feet and geese, are a result of this process. b A vowel which has been so altered, as ä, ö, and ü in German.
2. In German, the two dots (̈) put over a vowel modified by umlaut: short for umlaut–mark.
—v.t. To modify by umlaut or mutation. [<G, change of sound <um about + laut sound]