umlaut

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A change in a vowel sound caused by partial assimilation especially to a vowel or semivowel occurring in the following syllable.
• noun A vowel sound changed in this manner.
• noun The diacritic mark (¨) placed over a vowel to indicate an umlaut, especially in German.
• transitive verb To modify by umlaut.
• transitive verb To write or print (a vowel) with an umlaut.

from The Century Dictionary.

• In philology, to form with the umlaut, as a form; also, to affect or modify by umlaut, as a sound.
• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun linguistics An assimilatory process whereby a vowel is pronounced more like a following vocoid that is separated by one or more consonants.
• noun 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).
• noun linguistics A vowel so assimilated.
• noun orthography The diacritical mark ( ¨ ) placed over a vowel, usually when it indicates such assimilation.
• verb To place an umlaut over a vowel.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a diacritical mark (two dots) placed over a vowel in German to indicate a change in sound