from
The Collaborative International Dictionary of English v.0.48
umlaut \um"laut\ ([=oo]m"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, m[aum]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.
[1913 Webster]