umlaut

1. The euphonic modification of a root vowel sound by the influence of a, u, or especially i, in the syllable which formerly followed.
   

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.