Umlaut

Definition
Umlaut is the phonological process in which back vowels or diphthongs are fronted due to (a) a following front vowel, or (b) a specific morpheme or morphological class.

Example
A well-known example is German umlaut. Plural nouns can be formed in several (unpredictable) ways. One possibility is fronting of the base vowel: Apfel:Äpfel and Bauch:Bäuche.

Links

 * Utrecht Lexicon of Linguistics

Other Languages
German Umlaut (de)