\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.