noun

• (phonology) The substitution of one root vowel for another, thus indicating a corresponding modification of use or meaning; vowel permutation, distinct from the phonetic influence of a succeeding vowel.
  "I declare to you that Literature was not written for schoolmasters, nor for schoolmistresses. I would not exchange it for a wilderness of schoolmasters. It should be delivered from them, who, with their silly Ablauts and ‘tendencies,’ can themselves neither read nor write."

verb

• (intransitive, linguistics, of a vowel-containing linguistic component) To undergo a change of vowel.
  "This root must once have ablauted, given the associated nominal derivatives prthii- 'broad', prthivl- 'earth'. However, it does not ablaut at all in its verbal forms."
• (transitive, linguistics) To cause to change a vowel.