umlaut

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A change in a vowel sound caused by partial assimilation especially to a vowel or semivowel occurring in the following syllable.
2. n. A vowel sound changed in this manner. Also called vowel mutation.
3. n. The diacritic mark (¨) placed over a vowel to indicate an umlaut, especially in German.
4. v. To modify by umlaut.
5. v. To write or print (a vowel) with an umlaut.

Century Dictionary and Cyclopedia

1. n. In philology, the German name, invented by Grimm, for a vowel-change in the Germanic languages, brought about by the influence of a vowel in the succeeding syllable: namely, of the vowel i, modifying the preceding vowel in the direction of e or i, and of the vowel u, modifying the preceding vowel toward a or u. Only the former, or the change by a following i (now generally lost or altered), is found in English or German: thus, German mann, männer; fall, fällen; maus, mäuse; fuss, füsse; etc.; in English the phenomena are only sporadic remains, like man, men; fall, fell; mouse, mice; foot, feet. In Icelandic both kinds of umlaut are frequent and regular changes. An English name sometimes used for ‘umlaut’ is mutation. Compare ablaut.
2. In philology, to form with the umlaut, as a form; also, to affect or modify by umlaut, as a sound.

Wiktionary

1. n. linguistics An assimilatory process whereby a vowel is pronounced more like a following vocoid that is separated by one or more consonants.
2. n. linguistics The umlaut process (as above) that occurred historically in Germanic languages whereby back vowels became front vowels when followed by syllable containing a front vocoid (e.g. Germanic lūsi > Old English līs(i) > Modern English lice).
3. n. linguistics A vowel so assimilated.
4. n. orthography The diacritical mark ( ¨ ) placed over a vowel, usually when it indicates such assimilation.
5. v. To place an umlaut over a vowel.

GNU Webster's 1913

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

WordNet 3.0

1. n. a diacritical mark (two dots) placed over a vowel in German to indicate a change in sound