a phonetic change in the timbre of the vowels a, o, and u under the influence of the vowel i in the following syllable. In the Germanic languages, this change is influenced by certain other vowels in the following syllable as well. The term “umlaut” was introduced by J. Grimm.
The umlaut is a regressive assimilation of vowels. The conditioned variants occurring as a result of the umlaut may become independent phonemes, and their alternation (the grammatical umlaut) may become a morphological feature. For example, in modern German the umlaut is often a grammatical means for forming the plural of nouns, both when the environment determining the conditioned variant is absent and in many analogous forms, as in Gast-Gäste and Mutter-Mütter.