/oom"lowt/ , Ling.
n.
1. a mark (¨) used as a diacritic over a vowel, as ä, ö, ü, to indicate a vowel sound different from that of the letter without the diacritic, esp. as so used in German. Cf. dieresis .
2. (in Germanic languages) assimilation in which a vowel is influenced by a following vowel or semivowel; apophony.
v.t.
3. to modify by umlaut.
4. to write an umlaut over.
[ 1835-45; um- about (i.e., changed) + Laut sound ]