Source : Webster's Revised Unabridged Dictionary (1913)

Umlaut \Um"laut\, 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.

   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
         ["a], ["o], ["u], or ae, oe, ue; as, m["a]nner or
         maenner, men, from mann, man. Examples of forms
         resulting from umlaut in English are geese pl. of
         goose, men pl. of man, etc.

Source : WordNet®

umlaut
     n : a diacritical mark (two dots) placed over a vowel in German
         to indicate a change in sound [syn: {dieresis}, {diaeresis}]