(in Indo-European languages) regular alternation in the internal phonological structure of a word element, especially alternation of a vowel, that is coordinated with a change in grammatical function or combination, as in English sing, sang, sung, song; apophony.
Origin of ablaut
1840–50; < German, equivalent to ab- off + Laut sound
"systematic vowel alteration in the root of a word to indicate shades of meaning or tense," a characteristic of Indo-European languages, 1849, from German Ablaut, literally "off-sound," coined by J.P. Zweigel in 1568 from ab "off" + Laut "sound, tone," from Old High German hlut (see listen). Popularized by Jacob Grimm.