Definition of umlaut
a mark ( ¨ ) used over a vowel, as in German or Hungarian, to indicate a different vowel quality, usually fronting or rounding. (especially in Germanic languages) the process by which a back vowel becomes front in the context of another front vowel, resulting, e.g., in the differences between modern German Mann and Männer or (after loss of the inflection) English man and men.
verb [with object]
modify (a form or a sound) by using an umlaut.