A vowel change, characteristic of Indo-European languages, that accompanies a change in grammatical function; for example, i, a, u in sing, sang, sung.
Patterned change of base vowels in forms of a word or in related words to show changes in tense, meaning, etc., as in the Indo-European languages (Ex.: drink, drank, drunk)
(intransitive, linguistics, of a vowel-containing linguistic component) To undergo a change of vowel.
(linguistics) To cause to change a vowel.
Origin of ablaut
From American Heritage Dictionary of the English Language, 5th Edition