[1] : The Collaborative International Dictionary of English v.0.48

umlaut \um"laut\ =oom"lout, n. g., from um about + laut
   sound. philol.
   the euphonic modification of a root vowel sound by the
   influence of a, u, or especially i, in the syllable which
   formerly followed.
   1913 webster

   note: it is peculiar to the teutonic languages, and was
         common in anglo-saxon. in german the umlauted vowels
         resulting from a, o, u, followed by old i, are written
         aum, "o, "u, or ae, oe, ue; as, maumnner or
         maenner, men, from mann, man. examples of forms
         resulting from umlaut in english are geese pl. of
         goose, men pl. of man, etc.
         1913 webster

[2] : WordNet (r) 2.0

umlaut
     n : a diacritical mark two dots placed over a vowel in german
         to indicate a change in sound syn: dieresis, diaeresis
see also:
dieresis diaeresis 

Umlaut >> U Words
Umlaut, definition of term: Umlaut
umlaut_pag1.html