Found:

Umlaut

PHONOLOGY/MORPHOLOGY: 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.

LIT.	Lieber, R. (1980) Spencer, A. (1991)