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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.

[German : um-, around, alteration (from Middle High German umb-, from umbe, from Old High German umbi; see ambhi in Indo-European roots) + Laut, sound (from Middle High German lūt, from Old High German hlūt; see kleu- in Indo-European roots).]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.