ablaut

ablaut \ABH-lout\, noun:

the systematic substitution of one root vowel sound for another in different inflectional forms or derivatives of a word, as in ring, rang, rung

Any discussion of the grammar and phonology of Proto-Indo-European (PIE) would be incomplete without reference to Ablaut (German for 'vowel gradation').
-- Douglas Simms, University of Texas at Austin

by 1849, from German Ablaut, literally, "off-sound," from ab "off" + Laut "sound, tone," from Old High German hlut. It was popularized by Jacob Grimm.