noun

• (linguistics) An assimilatory process whereby a vowel is pronounced more like a following vocoid that is separated by one or more consonants.
  "In fits of concealed despair that went unnoticed even by those close enough to touch, Julien cursed the language of umlauts, eszetts, and gerunds."
• (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 lūsiz > Old English lȳs(i) > Modern English lice).
• (linguistics) A vowel so assimilated.
• (orthography) The diacritical mark ( ¨ ) placed over a vowel when it indicates a (rounded) front vowel

verb

• (transitive) To place an umlaut over (a vowel).
  "We kept some of the foreign alphabet symbols such as the accented and umlauted vowels and Greek letters and used the rest of the space for more esoteric linguistic symbols such as "barred-l," and "engma.""
• (linguistics, transitive) To modify (a word) so that an umlaut is required in it.
  "an umlauting vowel"