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. Compare dieresis.
2.
Also called vowel mutation.(in Germanic languages) assimilation in which a vowel is influenced by a following vowel or semivowel.
–verb (used with object)
3.
to modify by umlaut.
4.
to write an umlaut over.
Origin: 1835–45; < G, equiv. to um- about (i.e., changed) + Laut sound
A change in a vowel sound caused by partial assimilation especially to a vowel or semivowel occurring in the following syllable.
A vowel sound changed in this manner. Also called vowel mutation.
The diacritic mark (¨) placed over a vowel to indicate an umlaut, especially in German.
tr.v.
um·laut·ed, um·laut·ing, um·lauts
To modify by umlaut.
To write or print (a vowel) with an umlaut.
[German : um-, around, alteration (from Middle High German umb-, from umbe, from Old High German umbi; see ambhi in Indo-European roots) + Laut, sound (from Middle High German lūt, from Old High German hlūt; see kleu- in Indo-European roots).]
[oo
m-lout] 
Linguistics