Merriam-Webster Collegiate Dictionary - umlaut

 I. noun  Etymology: German, from um- around, transforming + Laut sound  Date: circa 1845  1.  a. the change of a vowel (as u to e in goose, geese) that is caused by partial assimilation to a succeeding sound or that occurs as a reflex of the former presence of a succeeding sound which has been lost or altered  b. a vowel resulting from such partial assimilation  2. a diacritical mark ? placed over a vowel to indicate a more central or front articulation — compare diaeresis  II. transitive verb  Date: 1879  1. to produce by ~  2. to write or print an ~ over