umlaut

• POS: Noun
• HYPHEN: um=laut

CIDE DICTIONARY

umlaut, n. [G., from um about + laut sound.].

 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]
" 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 ä, ö, Ü, or ae, oe, ue; as, männer 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]

OXFORD DICTIONARY

umlaut, n. & v.
--n.
1 a mark used over a vowel, esp. in Germanic languages, to indicate a vowel change.
2 such a vowel change, e.g. German Mann, M{auml}nner, English man, men, due to i, j, etc. (now usu. lost or altered) in the following syllable.
--v.tr. modify (a form or a sound) by an umlaut.

Etymology

G f. um about + Laut sound