noun

1. 1. a historical change in the sound of a vowel, caused by its assimilation to another vowel or semivowel originally occurring in the next syllable but later sometimes lost; mutation: in English, the differences of vowel in certain singulars and plurals (Ex.: foot—feet, mouse—mice) or in certain causative verbs and the words from which they are derived (Ex.: gold—gild) are due to the effects of umlaut on the second word of each pair
   2. a vowel resulting from such assimilation
2. the diacritical mark (¨) placed over a vowel, esp. in German, to indicate umlaut

Etymology: Ger, change of sound (< um, about + laut, sound, akin to loud): coined (1774) by F. G. Klopstock (1724-1803), Ger poet, but first used in special senses by Jakob Jakob (Ludwig Karl) Grimm (1819)

transitive verb