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]

WordNet (r) 3.0 (2006):

umlaut
    n 1: a diacritical mark (two dots) placed over a vowel in German
         to indicate a change in sound [syn: umlaut, dieresis,
         diaeresis]