Definition of "umlaut" []

• A change in a vowel sound caused by partial assimilation especially to a vowel or semivowel occurring in the following syllable. (noun)
• A vowel sound changed in this manner. Also called vowel mutation. (noun)
• The diacritic mark (¨) placed over a vowel to indicate an umlaut, especially in German. (noun)
• To modify by umlaut. (verb-transitive)
• To write or print (a vowel) with an umlaut. (verb-transitive)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "What happens in umlaut is that a back vowel is modified so as to have the form of the corresponding front vowel when there is a front vowel in the following syllable; this typically happens in plural forms of nouns, comparative forms of adjectives, and other words that have suffixes, so Mann (man) becomes Männer (men), lang (long) becomes länger (longer), and Tod (death) becomes tödlich (deathly, lethal)."
• "The preposition um means around or surrounding, but as a prefix the word has the idea of changing or modifying; laut means sound, so an umlaut is a modified sound."
• "Now an umlaut is masculine, but an accent mark ...?"