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a. A change in a vowel sound caused by partial assimilation especially to a vowel or semivowel occurring in the following syllable.
b. A vowel sound changed in this manner. Also called vowel mutation.
2. The diacritic mark (¨) placed over a vowel to indicate an umlaut, especially in German.
tr.v. um·laut·ed, um·laut·ing, um·lauts
1. To modify by umlaut.
2. To write or print (a vowel) with an umlaut.