ablaut (n.)
"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.