Ablaut n. (Philol.) The substitution of one root vowel for another, thus indicating a corresponding modification of use or meaning; vowel permutation; as, get, gat, got; sing, song; hang, hung.
... related to one another in different ways, the most important of which are mutation (umlaut), fracture (brechung), and gradation (ablaut). ... — An Icelandic Primer - With Grammar, Notes, and Glossary • Henry Sweet
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