ETYM German, off-sound; ab off + laut sound.
A vowel whose quality or length is changed to indicate linguistic distinctions (such as sing sang sung song); SYN. gradation.
variation in root vowel of words to change meaning.
special change or relation in vowel sound, as in vowels of “ring, rang, rung”.
A systematic variation of vowels in the same root or affix or in related roots or affixes especially in the Indo-European languages that is usually paralleled by differences in use or meaning (as in sing, sang, sung, song).

ablaut.