Umlaut
in philology, a kind of assimilation of sounds; the change of the vowel in one syllable through the influence of one of the vowels, a, i, u in the syllable immediately following. It is a common feature in several of the Teutonic tongues. In German umlaut is seen in the frequent change of the vowels a, o, u, to ä, ö, ü. In Anglo-Saxon it was also common. The change caused by a is called a-umlaut, and so of the other vowels.