n. & v.
n. 1 a mark used over a vowel, esp. in Germanic languages, to indicate a vowel change. 2 such a vowel change, e.g. German Mann, Mnner, English man, men, due to i, j, etc. (now usu. lost or altered) in the following syllable.
v.tr. modify (a form or a sound) by an umlaut.
[ G f. um about + Laut sound ]